Mend Your Misunderstanding of Multiple Myeloma Coding
Hint: ICD-10-CM coding for smoldering multiple myeloma isn’t what you’d expect. Although multiple myeloma (MM), smoldering multiple myeloma (SMM), and monoclonal gammopathy of undetermined significance (MGUS) are related plasma-cell disorders, they are not interchangeable. So, it’s important that as a coder, you understand their clinical differences, because unclear or incomplete documentation can result in coding confusion and inaccurate code assignment. In this first part of a two-part series, you’ll learn how to code MM, SMM, and MGUS, and you’ll see how ICD-10-CM looks at one of these conditions in a way that differs with prevailing clinical wisdom. First, Understand the Difference Between the Conditions Multiple myeloma is a systemic blood cancer that develops in plasma cells, a type of white blood cell found in the bone marrow. As abnormal plasma cells multiply, they can crowd out healthy blood-forming cells and damage or weaken bones. Bone involvement may occur in multiple areas, including the spine, skull, pelvis, rib cage, shoulders and hips. It occurs more commonly in older adults and is the second most common type of blood cancer in the United States. Some patients experience symptoms, while others are diagnosed before symptoms develop. The principal diagnostic evaluations include blood and urine tests to detect monoclonal protein (M protein) and related abnormalities; imaging studies to identify bone or bone marrow lesions; and bone marrow aspiration and biopsy to determine the presence and percentage of abnormal plasma cells. Cytogenetic testing, including fluorescence in situ hybridization (FISH), may also identify genetic abnormalities that help determine risk and prognosis. MM’s cause is not fully understood. Although MM is treatable and advances in therapy have improved outcomes, it is not currently considered curable. Smoldering multiple myeloma is also known as smoldering myeloma and asymptomatic myeloma. Medical sources describe SMM using different terminology. The International Myeloma Foundation describes it as “an asymptomatic precursor” to multiple myeloma, while other sources describe it as “an early form of myeloma that does not cause any symptoms or problems.” SMM may share certain findings with MM, including increased abnormal plasma cells in the bone marrow, monoclonal protein in the blood, and monoclonal light chains — also called Bence Jones protein — in the urine. However, SMM has no myeloma-defining events or related organ damage. It is considered an intermediate plasma-cell condition between MGUS and active MM. Monoclonal gammopathy of undetermined significance is a benign, noncancerous plasma-cell disorder and a possible precursor to SMM or MM. In MGUS, abnormal plasma cells produce multiple copies of the same antibody, called a monoclonal protein or M protein. These cells make up less than 10 percent of the bone marrow and do not cause myeloma-defining events or related organ damage, such as hypercalcemia, renal impairment, anemia, or bone lesions. MGUS is often discovered incidentally when a routine blood test identifies an elevated protein level, and further testing confirms the presence of an M protein. If abnormal plasma cells continue to accumulate and acquire additional biological abnormalities, MGUS may progress to SMM and, in some patients, eventually to active MM. However, most patients with MGUS do not develop MM. The average risk of progression is approximately 1 percent per year. Medical sources do not always describe SMM in the same way. While the International Myeloma Foundation sees SMM as “an asymptomatic precursor” of MM, others, such as the Hematology-Oncology Associates of CNY, view SMM as “the earliest stage of multiple myeloma,” during which patients may be symptom-free and have no signs of bone damage. These differing clinical descriptions help explain why SMM may create confusion for coders and others. Then, Understand Why SMM Creates Coding Confusion The terminology used to describe SMM creates a difficult coding question: Is SMM an early form or stage of MM, or is it an asymptomatic precursor condition that has not progressed to active MM? The 2026 ICD-10-CM Alphabetic Index lists Myeloma (multiple) as a malignant neoplasm under C90.0- (Multiple myeloma). The Tabular List then lists multiple myeloma and plasma-cell myeloma as inclusion conditions for C90.0- but excludes SMM, solitary myeloma, and solitary plasmacytoma. To document SMM, you’ll need to follow AHA Coding Clinic® for ICD-10-CM and ICD-10-PCS (Volume 8, Number 3, 2021) advice, which tells you to assign D47.2 (Monoclonal gammopathy) — the same nonmalignant code category used for MGUS. In other words, the AHA Coding Clinic® views SMM as a plasma-cell disorder that has not progressed to active MM and is not classified as a malignant neoplasm. This creates an apparent conflict or misunderstanding for coders that requires clear and specific provider documentation in order to distinguish the condition from active MM, MGUS, and solitary myeloma, which you would code with C90.3- (Solitary plasmacytoma). Test yourself: The oncologist documents SMM and plans continued laboratory monitoring without treatment. Although the diagnosis contains the words multiple myeloma, the AHA Coding Clinic® advice directs the coder to D47.2 rather than C90.00 (Multiple myeloma not having achieved remission). Last, Get to Know the Other Multiple Myeloma Codes The C90.0- codes are further subdivided into remission and relapse categories as follows: The Key Takeaway Although most sources agree that MGUS, SMM, and active MM represent different points along the plasma-cell disease spectrum, ICD-10-CM does not provide a distinct, malignant diagnosis code for SMM or high-risk SMM. This lack of specificity has become more significant following Food and Drug Administration (FDA) approval in November 2025 of treatment for high-risk SMM. A separate code could improve documentation, coding accuracy, research data, quality measurement, and recognition of the resources required for monitoring and treatment. But until ICD-10-CM adds such a code, you’ll have to follow the AHA Coding Clinic® advice for coding SMM cited earlier. Why is this important? Under the current Centers for Medicare & Medicaid Services (CMS) risk adjustment model, D47.2 does not map to a hierarchical condition category (HCC). HCCs are used by CMS and health plans in value-based care to predict healthcare costs and calculate risk-adjusted payments. In contrast, when a provider documents multiple myeloma using C90.00, the code maps to an HCC. Consequently, coding SMM as D47.2 may underrepresent the patient’s clinical condition and anticipated healthcare needs. This may affect risk-adjusted reimbursement and the resources available to support appropriate monitoring and treatment. This is part 1 of a two-part series. Part 2 will address MM remission and history status and metastatic disease coding in detail. Delly Parham, CRC, CPMA, CPC, Risk Adjustment Coder
