Overview

The Revised International Staging System (R-ISS) is the internationally validated prognostic framework for stratifying newly diagnosed symptomatic multiple myeloma (MM). Published in 2015 by Palumbo, Avet-Loiseau, and colleagues on behalf of the International Myeloma Working Group (IMWG), the R-ISS was derived and validated on a pooled dataset of 4,445 newly diagnosed patients from 11 multinational clinical trials. It superseded the original International Staging System (ISS, 2005) by adding two biologically and prognostically independent variables to the established ISS framework: chromosomal abnormalities (CA) detected by interphase fluorescence in situ hybridisation (iFISH) and serum lactate dehydrogenase (LDH).

The R-ISS generates three stages (I, II, III) with meaningfully distinct survival benchmarks from the IMWG derivation cohort: a 5-year overall survival (OS) of 82% in Stage I, 62% in Stage II, and 40% in Stage III, with corresponding median progression-free survival (PFS) values of 66, 42, and 29 months respectively. These estimates provide a structured evidence base for patient counselling, treatment intensity decisions, and clinical trial stratification in newly diagnosed MM.

Multiple Myeloma: Disease Background

Multiple myeloma is a clonal plasma cell malignancy characterised by the accumulation of abnormal immunoglobulin-secreting plasma cells in the bone marrow, producing end-organ damage defined by the CRAB criteria: hypercalcaemia, renal impairment, anaemia, and bone lesions. It is the second most common haematological malignancy in most Western registries, with approximately 35,000 new diagnoses annually in the United States and an estimated global incidence of 160,000 cases per year.

Myeloma has a median age at diagnosis of approximately 69 years, with fewer than 2% of cases occurring before age 40. The disease is preceded in essentially all cases by a precursor state, either monoclonal gammopathy of undetermined significance (MGUS) or smouldering multiple myeloma (SMM), though the time spent in these precursor phases is highly variable and often unrecognised. Symptomatic myeloma requiring treatment is defined by the IMWG 2014 diagnostic criteria, which expanded the traditional CRAB criteria to include three biomarkers of malignancy (clonal bone marrow plasma cells ≥60%, serum free light chain ratio ≥100, and more than one focal lesion on MRI) to allow earlier diagnosis before overt end-organ damage occurs.

The biological landscape of MM is highly heterogeneous. The initiating oncogenic event in most cases is an immunoglobulin heavy-chain translocation involving chromosome 14q32, or hyperdiploidy characterised by gains of multiple odd-numbered chromosomes. These primary genetic events are followed over time by secondary mutations involving RAS/MAPK signalling (NRAS, KRAS, BRAF), NF-κB pathway activation, MYC dysregulation, and ultimately the deletion or mutation of TP53 in advanced disease. The R-ISS captures a portion of this genomic complexity through its iFISH-based cytogenetic risk grouping.

Treatment of newly diagnosed MM has been transformed over the past two decades by the introduction of proteasome inhibitors (bortezomib, carfilzomib, ixazomib), immunomodulatory drugs (thalidomide, lenalidomide, pomalidomide), and anti-CD38 monoclonal antibodies (daratumumab, isatuximab). Current frontline induction for transplant-eligible patients typically consists of a quadruplet regimen incorporating an anti-CD38 antibody with a proteasome inhibitor, an IMiD, and dexamethasone, followed by autologous stem cell transplantation (ASCT) and maintenance lenalidomide. For transplant-ineligible patients, continuous triplet or quadruplet regimens without ASCT are standard. Despite these advances, myeloma remains largely incurable in the majority of patients, underscoring the continued importance of risk stratification tools that guide treatment intensity and monitoring frequency.

Historical Staging: From ISS to R-ISS

The Original International Staging System (2005)

The original ISS, published by Greipp et al. in Journal of Clinical Oncology in 2005, was derived from a dataset of 10,750 myeloma patients across 17 institutions in North America, Europe, and Asia. It identified two serum proteins as the most powerful independent prognostic variables: serum β2-microglobulin (Sβ2M) and serum albumin. Their combination generated a simple three-stage system:

• ISS I: Sβ2M <3.5 mg/L and serum albumin ≥3.5 g/dL
• ISS III: Sβ2M ≥5.5 mg/L
• ISS II: Neither ISS I nor ISS III

The ISS was globally adopted as the standard prognostic instrument for clinical practice and trial stratification. Its simplicity and reproducibility were key strengths: both β2-microglobulin and albumin are measured by routine chemistry assays in any clinical laboratory worldwide without specialist equipment. Median survival in the ISS derivation cohort was 62 months for Stage I, 44 months for Stage II, and 29 months for Stage III, in an era predating widespread proteasome inhibitor use.

The primary limitation of the ISS was its exclusive reliance on tumour-burden and nutritional markers, with no direct measure of myeloma biology or genomic aggressiveness. Two patients with identical ISS stages could have fundamentally different disease behaviours based on their underlying cytogenetics: a patient with ISS II disease and del(17p) has a dramatically worse prognosis than a patient with ISS II disease and a normal karyotype, yet the ISS treated them identically. This limitation became increasingly evident as cytogenetic risk stratification entered routine clinical practice following validation of iFISH as a prognostic tool in multiple international datasets during the 2000s.

Development of the R-ISS (2015)

The IMWG R-ISS working group addressed the ISS’s biological agnosticism by systematically evaluating which additional variables, beyond Sβ2M and albumin, most significantly improved prognostic discrimination. Cox regression analyses in the pooled 4,445-patient dataset identified two independently prognostic additions: iFISH-detected high-risk chromosomal abnormalities and elevated serum LDH.

Critically, both variables were retained as binary classifiers (present or absent) rather than continuous variables, ensuring that the R-ISS remained simple enough for routine clinical application. The resulting staging algorithm is a two-step hierarchical classification: first assign ISS stage from the existing two-variable rule, then apply cytogenetic and LDH findings to either upgrade (ISS I to R-ISS II if adverse biology is present) or further stratify (ISS III to R-ISS III if adverse biology is present, or to R-ISS II if biology is standard).

The Four Variables: Biology and Clinical Significance

1. Serum β2-Microglobulin (Sβ2M)

β2-microglobulin is an 11.8 kDa protein that forms the non-covalent light chain of MHC class I molecules on the surface of essentially all nucleated cells. It is shed constitutively from cell membranes and cleared by glomerular filtration, with subsequent catabolism in the proximal renal tubule. Serum β2M concentrations therefore reflect two independent biological processes in myeloma:

1. Tumour burden: Elevated plasma cell mass produces more surface MHC I and consequently more shed β2M. Higher plasma cell burden correlates directly with serum β2M in the absence of renal impairment.
2. Renal function: Impaired glomerular filtration reduces β2M clearance, raising serum levels independently of tumour burden. Myeloma patients with cast nephropathy, light-chain deposition disease, or amyloidosis may have markedly elevated β2M disproportionate to their tumour mass.

Despite this dual biological origin, serum β2M remains the single most powerful prognostic variable in MM across virtually all published staging analyses, outperforming M-protein level, bone marrow plasma cell percentage, and albumin individually. Its three-tier categorisation in the R-ISS (via the ISS) uses thresholds of <3.5 mg/L, 3.5 to <5.5 mg/L, and ≥5.5 mg/L, derived from the original ISS optimisation analysis as the cut-points that maximise prognostic separation.

In practice, serum β2M should be measured at diagnosis before initiation of any disease-modifying therapy or renal support. In patients presenting with acute renal failure, reassessment of β2M after renal recovery or dialysis initiation may be informative for revising prognosis, though formal R-ISS re-staging in this context is not standardised.

2. Serum Albumin

Serum albumin contributes to R-ISS staging solely through its role in ISS stage determination (specifically, differentiating ISS Stage I from Stage II when β2M is below 3.5 mg/L). Low albumin (<3.5 g/dL) in the context of myeloma may arise from multiple mechanisms:

• Interleukin-6 (IL-6) excess: IL-6 is the primary growth factor for myeloma plasma cells and a key cytokine in the bone marrow microenvironment. IL-6 is also a potent suppressor of hepatic albumin synthesis; elevated IL-6 levels from the tumour microenvironment directly reduce albumin production, making hypoalbuminaemia in MM partly a marker of IL-6-driven tumour-host crosstalk.
• Nutritional depletion: Systemic illness, constitutional symptoms, and anorexia associated with advanced MM may reduce dietary protein intake.
• Protein-losing states: Amyloid-associated nephrotic syndrome or gastrointestinal AL amyloidosis occasionally contributes to hypoalbuminaemia in MM patients with light-chain amyloidosis.

In patients with concurrent inflammatory illness at diagnosis (e.g. severe infection), albumin may be acutely suppressed as an acute-phase response, with recovery following treatment. Where possible, the albumin value used for R-ISS staging should represent a steady-state measurement from the diagnostic workup rather than an acute illness nadir.

3. Chromosomal Abnormalities by iFISH

Interphase fluorescence in situ hybridisation (iFISH) on CD138-enriched bone marrow plasma cells is the reference-standard method for cytogenetic risk stratification in MM. Unlike conventional metaphase cytogenetics, iFISH does not require dividing cells, making it more sensitive in a typically low-proliferation tumour. CD138 (syndecan-1) selection enriches the plasma cell fraction from typically 2 to 5% in an unmanipulated aspirate to 85 to 95%, dramatically improving the sensitivity for detecting abnormalities present in the malignant clone.

The R-ISS defines high-risk cytogenetics as the presence of any one or more of three specific chromosomal abnormalities:

• del(17p): Deletion of the short arm of chromosome 17, encompassing the TP53 tumour suppressor locus. Del(17p) is the most uniformly adverse cytogenetic event in myeloma and is detected in approximately 8 to 12% of newly diagnosed patients. Loss of one TP53 allele, particularly when combined with mutation of the retained allele (biallelic TP53 inactivation), drives genomic instability, chemotherapy resistance, and aggressive disease biology. In the era of novel agents, del(17p) remains a dominant adverse prognostic factor in virtually all subgroup analyses, though its absolute impact is modestly attenuated by contemporary quadruplet regimens compared with older triplets.
• t(4;14)(p16;q32): This translocation juxtaposes the FGFR3 (fibroblast growth factor receptor 3) and MMSET/NSD2 genes at chromosome 4p16 with the immunoglobulin heavy chain (IGH) locus at 14q32. MMSET overexpression drives global H3K36 dimethylation changes, broadly dysregulating gene expression and increasing proliferative signalling. T(4;14) is found in approximately 15% of newly diagnosed patients and defines a subtype with intermediate to high risk. Notably, bortezomib-containing regimens appear to partially overcome the adverse prognosis of t(4;14) compared with older non-bortezomib regimens, and this translocation is now considered intermediate rather than uniformly high risk by some expert groups in the context of modern therapy.
• t(14;16)(q32;q23): This translocation places the MAF proto-oncogene (a leucine-zipper transcription factor) under IGH enhancer control, driving constitutive MAF expression and consequent upregulation of cyclin D2 and integrin β7. T(14;16) is relatively uncommon, present in approximately 5% of newly diagnosed patients, and is consistently associated with poor outcomes across treatment eras. There is no specific targeted therapy for this translocation currently in routine clinical use, though antibody-drug conjugates and BCMA-directed therapies are being evaluated.

Approximately 20% of newly diagnosed myeloma patients harbour at least one R-ISS high-risk cytogenetic abnormality. The three abnormalities may co-occur (e.g. del(17p) superimposed on t(4;14) is a particularly adverse combination), and such compound high-risk genetics are associated with even worse outcomes than single high-risk abnormalities, a nuance not captured by the binary R-ISS classification.

A key technical requirement is that the iFISH panel must be performed on CD138-purified plasma cells. FISH results from unpurified bone marrow aspirate smears are subject to dilution by non-clonal haematopoietic cells and have lower sensitivity. Where CD138 selection is not available, interphase FISH on plasma cell-enriched preparations using cytoplasmic immunoglobulin staining (cIg-FISH) is an acceptable alternative. Results from conventional G-band karyotyping alone are insufficient and should not be used for R-ISS cytogenetic classification.

4. Serum Lactate Dehydrogenase (LDH)

Lactate dehydrogenase is a ubiquitous intracellular enzyme present in five tetrameric isoforms across virtually all tissues. It is released into the circulation following cell death or membrane damage from any cause, making it a non-specific but widely measurable marker of tissue injury and cell turnover.

In the context of multiple myeloma, elevated serum LDH above the institutional upper limit of normal (ULN) is a marker of aggressive tumour biology, correlating with:

• High tumour proliferative activity: The proportion of myeloma cells in S-phase (plasma cell labelling index, PCLI) correlates with serum LDH in clinical studies, as rapidly cycling cells have higher metabolic rates and greater susceptibility to necrosis.
• Extramedullary disease: Plasmacytomas and extramedullary myeloma, which have a more aggressive biology and poorer response to standard therapy, are associated with elevated LDH in a substantial proportion of cases.
• Secondary plasma cell leukaemia: Primary or secondary plasma cell leukaemia is almost uniformly associated with markedly elevated LDH and represents one of the most aggressive myeloma variants.
• High-risk cytogenetics: There is partial but incomplete correlation between LDH elevation and high-risk chromosomal abnormalities, justifying LDH as an independent rather than redundant variable in the R-ISS.

The R-ISS uses a simple binary classification: LDH normal (at or below the institutional ULN) versus LDH elevated (above ULN). The ULN varies between institutions and assay platforms; the value reported by the local laboratory should be used rather than a fixed universal threshold. Haemolysed samples falsely elevate LDH and should be redrawn before accepting an elevated result at diagnostic workup.

R-ISS Staging Algorithm

The R-ISS operates as a two-step hierarchical classification. The first step determines the ISS stage from serum β2-microglobulin and albumin alone. The second step incorporates cytogenetic risk and LDH to determine the final R-ISS stage:

Step 1: Determine ISS Stage

Step 2: Determine R-ISS Stage

This hierarchical structure means that high-risk cytogenetics or elevated LDH alone are not sufficient to assign R-ISS Stage III; ISS Stage III must also be present. Similarly, ISS Stage I with adverse biological features is not R-ISS Stage I: the presence of high-risk cytogenetics or elevated LDH in an otherwise ISS I patient assigns them to R-ISS Stage II. This reflects the statistical interaction between tumour burden markers and biological aggressiveness in determining outcome.

Prognostic Outcomes by R-ISS Stage

The following outcome benchmarks are derived from the IMWG validation cohort of 4,445 newly diagnosed symptomatic myeloma patients treated across 11 international clinical trials with regimens including thalidomide-, bortezomib-, and lenalidomide-based induction with or without ASCT:

Several observations are important for correct interpretation of these figures:

• The survival estimates represent statistical medians from a clinical trial population treated in 2003 to 2013. Contemporary induction with quadruplet regimens and improved maintenance strategies have substantially improved outcomes across all stages, and the absolute figures should not be quoted to patients as current expected survival. They are most informative for relative risk comparisons across stages and for clinical trial design and stratification.
• The R-ISS Stage II category is the largest (62% of patients) and the most heterogeneous. It encompasses ISS II patients with standard biology (who may behave closer to Stage I) and ISS I patients with one adverse biological feature (who may behave closer to Stage III). This within-stage heterogeneity is the most significant limitation of the R-ISS and is partly addressed by the subsequently developed R2-ISS.
• R-ISS Stage III constitutes only 10% of newly diagnosed patients, reflecting the conjunctive requirement for both ISS Stage III and adverse biology. Patients meeting only one of these criteria are classified as Stage II.

Clinical Management by R-ISS Stage

R-ISS Stage I: Favourable Prognosis

R-ISS Stage I patients have the most favourable prognosis and comprise approximately 28% of newly diagnosed symptomatic myeloma. Their combination of low tumour burden, standard cytogenetics, and normal LDH identifies a biologically indolent disease subtype with expected deep and durable responses to contemporary therapy.

The treatment approach for eligible patients follows standard frontline protocols: triplet or quadruplet induction incorporating a proteasome inhibitor (bortezomib) and an IMiD (lenalidomide) with dexamethasone, followed by ASCT in transplant-eligible patients. Lenalidomide maintenance post-ASCT has demonstrated OS benefit in landmark trials (CALGB 100104, IFM 2005-02) and is standard regardless of R-ISS stage in transplant-eligible patients.

Monitoring for Stage I patients involves serial M-protein quantification (serum protein electrophoresis and immunofixation, free light chains), full blood counts, renal function, calcium, and LDH at appropriate intervals. The excellent prognosis in this group means that time to measurable residual disease (MRD) negativity by next-generation sequencing or flow cytometry is an increasingly used surrogate endpoint that predicts long-term outcomes. MRD negativity in Stage I patients is associated with PFS outcomes that may extend 10 years or more in some series.

R-ISS Stage II: Intermediate Prognosis

R-ISS Stage II is the largest and most heterogeneous category, encompassing the majority (62%) of newly diagnosed patients. Its heterogeneity demands careful disaggregation of the underlying risk factors to guide treatment intensity:

• ISS II with standard cytogenetics and normal LDH: These patients occupy the biological middle ground of Stage II and typically respond well to standard induction. Treatment follows the same principles as Stage I, with standard triplet or quadruplet induction and ASCT in eligible patients.
• ISS I with high-risk cytogenetics or elevated LDH: These patients have low tumour burden but biologically aggressive disease. High-risk cytogenetics in particular (del(17p), t(4;14), t(14;16)) warrant consideration of intensified treatment strategies, including quadruplet induction, tandem ASCT in select patients (particularly t(14;16) or compound high-risk cytogenetics), and extended maintenance. Clinical trial enrollment is particularly valuable in this subgroup.
• ISS III with standard cytogenetics and normal LDH: High tumour burden without adverse biology. Standard induction is appropriate, with close attention to renal function management (which contributes to the elevated β2M) and aggressive supportive care.

The addition of anti-CD38 antibodies (daratumumab, isatuximab) to triplet induction regimens has improved depth of response and PFS across R-ISS stages in both transplant-eligible (CASSIOPEIA, GRIFFIN, PERSEUS trials) and transplant-ineligible (MAIA, IMROZ trials) patients, with particular benefit observed in cytogenetically high-risk subgroups within Stage II.

R-ISS Stage III: Poor Prognosis

R-ISS Stage III represents the highest-risk newly diagnosed myeloma patient, with the combination of high tumour burden (ISS Stage III: β2M ≥5.5 mg/L) and aggressive biological features (high-risk cytogenetics and/or elevated LDH). These patients have 5-year OS of approximately 40% and require the most intensive available treatment strategies.

Key management principles for R-ISS Stage III include:

• Quadruplet induction: The addition of daratumumab to VRd (bortezomib, lenalidomide, dexamethasone) in the GRIFFIN and PERSEUS trials demonstrated superior MRD negativity rates and significantly prolonged PFS compared with VRd alone, including in cytogenetically high-risk patients. Dara-VRd is now preferred over triplet induction in eligible patients regardless of stage, but is especially important in Stage III.
• ASCT consideration: All eligible Stage III patients should be offered ASCT consolidation after induction. Tandem ASCT has been explored in very high-risk patients, particularly those with del(17p) or compound high-risk cytogenetics, though the evidence base for this strategy remains incompletely defined.
• Extended or intensified maintenance: Lenalidomide maintenance is standard post-ASCT, but for Stage III patients with high-risk cytogenetics, combinations of lenalidomide with bortezomib or ixazomib may provide additional depth of response maintenance. This area remains under active investigation.
• Novel and investigational agents: Clinical trial enrollment is strongly encouraged for R-ISS Stage III patients. CELMoDs (mezigdomide, iberdomide), bispecific T-cell engager antibodies (teclistamab, talquetamab), and antibody-drug conjugates (belantamab mafodotin) are being evaluated in earlier lines and high-risk subgroups.
• Goals-of-care discussions: For Stage III patients who are transplant-ineligible due to age or comorbidities, realistic counselling about expected outcomes with available therapies is essential. Palliative care integration should be considered from diagnosis, concurrent with active myeloma therapy, in keeping with the evidence base supporting early palliative care in haematological malignancies.

R-ISS and Measurable Residual Disease (MRD)

The integration of MRD assessment with R-ISS staging represents an evolving frontier in myeloma prognostication. MRD negativity (defined as absence of clonal plasma cells at a sensitivity of 10⁻⁵ or deeper by next-generation sequencing or next-generation flow cytometry) is now the most powerful response endpoint predicting long-term outcomes, outperforming traditional response criteria (CR, VGPR, PR) in prognostic analyses.

The MRD status interacts with R-ISS stage in clinically meaningful ways. Among R-ISS Stage I patients who achieve MRD negativity, PFS outcomes are exceptional and may approach those of age-matched individuals without myeloma in long-term follow-up. Among R-ISS Stage III patients who achieve MRD negativity post-transplant, outcomes are substantially better than Stage III patients who remain MRD-positive, though still inferior to MRD-negative Stage I patients, indicating that cytogenetic biology influences durability of MRD negativity even when it is initially achieved.

Ongoing clinical trials are testing whether sustained MRD negativity can guide treatment de-escalation (e.g. maintenance discontinuation in Stage I MRD-negative patients) or intensification (e.g. earlier salvage in MRD-positive Stage III patients). The R-ISS will likely become embedded within composite MRD-based prognostic frameworks as this evidence matures.

The R2-ISS: A Second Revision (2022)

Recognising the heterogeneity of R-ISS Stage II and the growing importance of additional cytogenetic risk factors not included in the 2015 R-ISS, D’Agostino and colleagues, on behalf of the European Myeloma Network (EMN) HARMONY Project, published the Second Revision of the ISS (R2-ISS) in Journal of Clinical Oncology in 2022, based on 5,538 patients from six major European prospective trials.

The R2-ISS adds 1q gain (defined as ≥3 copies of chromosome 1q21, detected by iFISH) as a new cytogenetic risk factor, recognising its presence in approximately 35 to 40% of newly diagnosed patients and its consistent adverse prognostic impact across multiple datasets. The R2-ISS also assigns differential weights to cytogenetic risk factors:

• Del(17p), t(4;14), t(14;16): 1 point each
• Gain(1q): 0.5 points
• ISS Stage II: 1 point
• ISS Stage III: 1.5 points
• Elevated LDH: 1 point

The resulting R2-ISS generates a continuous score that maps to four risk groups (I, II, III, IV), subdividing the large R-ISS Stage II category. The R2-ISS demonstrates improved prognostic discrimination compared with R-ISS (higher C-statistic for both OS and PFS) and is beginning to appear in updated international guidelines as a recommended complement or successor to R-ISS in centres with comprehensive iFISH panels that include 1q assessment.

Importantly, R2-ISS does not obsolete R-ISS. The R-ISS remains the most widely validated, internationally adopted standard with the largest evidence base, and it continues to underpin eligibility and stratification criteria in most ongoing clinical trials. Where 1q21 iFISH is not routinely performed or the R2-ISS has not been institutionally adopted, R-ISS remains fully appropriate.

Comparison with Other MM Staging and Risk Systems

Durie-Salmon Staging (DSS)

The original Durie-Salmon Staging system (1975) was the first internationally adopted myeloma staging framework. It uses M-protein level, haemoglobin, serum calcium, bone lesion number, and urine light chain excretion to classify patients into three stages with a renal function subclassifier (A = creatinine <2 mg/dL, B = ≥2 mg/dL). The DSS correlates roughly with tumour cell mass (Stage I: <0.6 ×10¹² cells/m², Stage III: >1.2 ×10¹² cells/m²). While historically important and still in use in some older institutional protocols, the DSS has been largely supplanted by ISS and R-ISS due to its complexity, subjectivity in bone lesion grading, and weaker prognostic discrimination in the novel-agent era.

Gene Expression Profiling (GEP) Risk Scores

Gene expression profiling of CD138-purified plasma cells has been used at specialist centres to generate high-dimensional risk signatures. The University of Arkansas GEP70 score defines high-risk based on a 70-gene signature and identifies approximately 15% of patients with a particularly poor prognosis (median OS below 2 years with older therapies). GEP signatures capture a distinct and partially non-overlapping high-risk population compared with R-ISS: some GEP-high patients have favourable R-ISS stages, and vice versa. GEP is not universally available and has not been adopted in routine clinical practice outside specialist centres, though it is used in some US cooperative group trials (e.g. CoMMpass, ECOG-ACRIN) for research stratification.

IMWG Frailty Score

The IMWG Frailty Score (Palumbo et al. 2015, Facon et al. 2019) addresses patient-level rather than disease-level risk, incorporating age, comorbidities (Charlson Comorbidity Index), and functional status (ADL and IADL scores) to classify patients as fit, intermediate fit, or frail. Frailty status is a major determinant of treatment tolerance and intensity and is independent of R-ISS stage. A young R-ISS Stage III patient who is fully fit should be managed very differently from an 80-year-old frail R-ISS Stage II patient. Integrating R-ISS stage with frailty assessment is essential for individualised treatment planning.

Practical Considerations for Calculator Use

Timing of Assessment

R-ISS staging is validated exclusively for newly diagnosed, previously untreated symptomatic multiple myeloma at diagnosis. All four input values should be obtained from the diagnostic workup, prior to initiation of any systemic therapy including dexamethasone, bisphosphonates, or plasmapheresis. Post-treatment values do not correspond to the validated R-ISS outcome benchmarks and should not be used for R-ISS staging.

Interpreting iFISH Reports

When reviewing the iFISH report, the key cytogenetic features to identify are:

• Del(17p): Reported as deletion of locus 17p13.1 or TP53 deletion. Most laboratories use a threshold of ≥10% of plasma cells showing deletion as clinically significant. Some centres use ≥20% as the threshold for treatment- relevant del(17p); the R-ISS was not optimised around a specific threshold, and local laboratory thresholds should be applied.
• T(4;14): Reported as FGFR3/MMSET rearrangement, IgH/FGFR3 fusion, or t(4;14) by split-signal FISH. Note that 25 to 30% of t(4;14) cases have lost FGFR3 expression (del(4p16) affecting only the der(14) chromosome) and may only show the MMSET probe signal; these should still be classified as t(4;14)-positive.
• T(14;16): Reported as IGH/MAF rearrangement or t(14;16). This translocation is uncommon and may not be included in all routine iFISH panels; its absence from a panel that did not probe for it should be distinguished from a confirmed negative result.

LDH Measurement

Serum LDH should be compared with the institutional upper limit of normal (ULN) from the reporting laboratory, not a fixed universal value. Normal ranges differ between assay platforms and patient populations. Haemolysis falsely elevates LDH and any haemolysed sample should be repeated. Where LDH is mildly elevated (1.0 to 1.2 times ULN), clinical judgement should be applied, as borderline elevations may reflect non-myeloma causes (hepatic disease, recent bone marrow sampling, muscle injury). Repeat measurement is appropriate before accepting an elevated LDH as a meaningful myeloma biomarker in equivocal cases.

Applying R-ISS in Clinical Context

The R-ISS is one component of a comprehensive risk and treatment assessment for newly diagnosed myeloma. It should always be integrated with:

• Renal function: Severe renal impairment at diagnosis (creatinine >2 mg/dL or eGFR <30 mL/min) influences both the β2M level (confounded by reduced clearance) and the choice of induction regimen (bortezomib-based preferred over lenalidomide-based due to renal dosing considerations).
• Bone disease extent: Extensive lytic disease, pathological fractures, or vertebral collapse may require urgent orthopaedic or radiation oncology input independent of R-ISS stage.
• Extramedullary disease: Soft tissue plasmacytomas at diagnosis confer adverse prognosis not fully captured by R-ISS and may warrant more aggressive or investigational treatment approaches.
• Gain(1q) and additional cytogenetics: For patients where gain(1q) iFISH was performed, this result contributes to R2-ISS risk assignment and should be incorporated into risk communication alongside R-ISS.
• Patient age and frailty: Treatment intensity should always be calibrated to fitness, not R-ISS stage alone. A frail patient with R-ISS Stage III may tolerate only attenuated therapy, whereas a fit 65-year-old with Stage I may benefit from the most intensive available regimen.

How to Use This Calculator

The CalcMD R-ISS calculator requires four inputs from the diagnostic workup of a newly diagnosed, previously untreated, symptomatic multiple myeloma patient:

1. Serum β2-microglobulin: Select the appropriate range from the diagnostic serum chemistry panel. Use the pre-treatment value. If the patient has received prior dexamethasone for symptom management, note that this may modestly alter M-protein but does not significantly affect β2M; the diagnostic value is still appropriate for R-ISS calculation.
2. Serum albumin: Select ≥3.5 g/dL or <3.5 g/dL from the diagnostic biochemistry panel. Use the pre-treatment, steady-state value.
3. Chromosomal abnormalities by iFISH: Select Standard risk (none of del(17p), t(4;14), or t(14;16) detected) or High risk (one or more detected). This must be based on iFISH from CD138-purified or cIg-FISH-selected plasma cells. If iFISH results are pending, the calculator can be used with conventional karyotype with the understanding that sensitivity for t(4;14) and t(14;16) on metaphase cytogenetics is substantially lower.
4. Serum LDH: Select Normal (at or below institutional ULN) or Elevated (above institutional ULN). Confirm the result is not from a haemolysed sample and represents the myeloma biological state rather than a confounding comorbidity.

The calculator will display the ISS intermediate stage, the final R-ISS stage, the 5-year OS and median PFS benchmarks from the IMWG validation cohort, and an interpretive summary. These results are intended to support clinical communication and treatment planning for patients with newly diagnosed symptomatic multiple myeloma and should not be used in isolation without full integration of the clinical picture, patient preferences, and institutional treatment protocols.

Limitations

• Newly diagnosed symptomatic MM only: R-ISS is not validated for smouldering myeloma, MGUS, light-chain amyloidosis, Waldenström macroglobulinaemia, or relapsed/refractory disease. Applying it in these settings will produce outputs that do not correspond to validated outcome benchmarks.
• Treatment era gap: The validation cohort was treated from 2003 to 2013 with regimens predating daratumumab, carfilzomib, and modern quadruplet induction. Absolute survival figures underestimate current outcomes; the staging system retains its relative discriminatory validity, but absolute figures should be contextualised with contemporary data when counselling patients.
• Stage II heterogeneity: The largest stage (62% of patients) contains a wide range of risk profiles. Clinicians should look beyond the stage label to the underlying driver of Stage II classification and consider R2-ISS where gain(1q) data are available.
• No gain(1q) or t(14;20) incorporation: Gain of chromosome 1q (particularly 1q21 gain or amplification with ≥4 copies) is now one of the most common and clinically significant adverse cytogenetic events in myeloma, present in 35 to 40% of newly diagnosed patients. It is not captured in the R-ISS. The R2-ISS incorporates it; where 1q iFISH is available, the R2-ISS provides more refined risk stratification.
• No molecular genetics: Somatic mutations in TP53 (biallelic), KRAS, NRAS, and other oncogenes that influence outcome in MM are not part of the R-ISS. These molecular features are increasingly captured in next-generation sequencing panels at specialist centres and represent additional prognostic dimensions beyond R-ISS.
• β2M confounding by renal impairment: The dual determinants of β2M (tumour burden and renal function) mean that patients with heavy cast nephropathy may have disproportionately elevated β2M relative to their true tumour burden. A patient who is ISS III primarily due to renal-failure-associated β2M elevation but has good cytogenetics may have a different biological trajectory than a patient who is ISS III due to true high tumour burden. This is a recognised intrinsic limitation of β2M-based staging.
• iFISH platform and threshold variability: Different iFISH platforms, probe sets, and threshold definitions for positivity vary between centres and may affect cytogenetic classification reproducibility. Results should always be interpreted by a laboratory with accredited haematological cytogenetics expertise.