Multiple myeloma is diagnosed through a combination of blood tests, urine tests, bone marrow biopsy, and imaging scans. No single test confirms it on its own. Instead, doctors piece together results from several tests to determine whether abnormal plasma cells have accumulated in the bone marrow and whether they’re causing damage to the body. The full diagnostic workup typically takes one to two weeks from the first round of blood draws to final biopsy results.
Blood and Urine Tests Come First
The diagnostic process usually starts with blood work. A test called serum protein electrophoresis separates the proteins in your blood by size and electrical charge, producing a pattern that reveals whether one type of antibody protein is being overproduced. This abnormal protein, often called an M-protein or M-spike, is the hallmark of plasma cell disorders. If an M-protein is detected, a follow-up test called immunofixation identifies the exact type of antibody involved.
A separate blood test measures free light chains, which are fragments of antibodies circulating in your bloodstream. In healthy people, the ratio between the two types of light chains (kappa and lambda) stays relatively balanced. In myeloma, one type is dramatically overproduced. A ratio of 100 or higher between the involved and uninvolved light chain is so strongly associated with progression that it alone can help define a myeloma diagnosis, even before organ damage appears.
Urine collection over 24 hours checks for the same abnormal proteins being filtered through the kidneys. Some forms of myeloma produce light chains that show up more readily in urine than in blood, so both tests are important for catching the full picture.
Bone Marrow Biopsy Confirms the Diagnosis
A bone marrow biopsy is the most definitive step. During the procedure, a needle is inserted into the back of the hip bone to extract a small core of marrow tissue and a liquid sample. The procedure takes about 15 to 30 minutes and is done under local anesthesia, sometimes with light sedation. Most people describe it as uncomfortable pressure rather than sharp pain, though a brief aching sensation is common when the marrow is drawn out.
Pathologists examine the sample to count what percentage of the marrow is made up of clonal plasma cells, meaning identical copies of the same abnormal cell. The 10% threshold is the key dividing line. Below 10% with an M-protein present typically points to a precursor condition called MGUS (monoclonal gammopathy of undetermined significance), which may never need treatment. At or above 10%, the findings support a myeloma diagnosis. If plasma cells make up 60% or more of the marrow, that alone qualifies as a myeloma-defining event regardless of whether symptoms have developed yet.
Results from the biopsy generally come back within a few days, though some specialized tests sent to outside laboratories can take longer.
CRAB Criteria: Signs of Organ Damage
For years, a myeloma diagnosis required evidence that the abnormal plasma cells were actively harming the body. Doctors use the acronym CRAB to describe the four classic types of damage:
- Calcium elevation: blood calcium above 11 mg/dL, caused by bone breakdown releasing calcium into the bloodstream
- Renal insufficiency: kidney filtration rate below 40 mL per minute, or creatinine above 2 mg/dL, from abnormal proteins clogging the kidneys
- Anemia: hemoglobin below 10 g/dL, because crowded-out marrow can no longer produce enough red blood cells
- Bone lesions: one or more holes in the bone visible on imaging, where myeloma cells have eaten away at the skeleton
Any one of these, combined with 10% or more plasma cells in the marrow, is enough to confirm active myeloma. Many patients first come to medical attention because of unexplained back pain from bone lesions or fatigue from anemia, and the CRAB findings emerge during the initial workup.
Diagnosis Without Organ Damage
A major update to the diagnostic criteria in 2014 recognized that some patients without CRAB symptoms already have disease that will almost inevitably progress. Rather than waiting for organ damage, doctors can now diagnose myeloma based on three biomarkers, sometimes called the SLiM criteria:
- Sixty percent or more plasma cells in the bone marrow
- Light chain ratio of 100 or higher between the involved and uninvolved type
- More than one focal lesion on MRI (a concentrated area of abnormal cells in the bone)
Meeting any one of these thresholds, even without elevated calcium, kidney problems, anemia, or visible bone holes, now qualifies as active myeloma requiring treatment. This change was designed to catch the disease earlier and prevent irreversible damage to bones and kidneys.
Imaging to Assess Bone and Tissue
Imaging plays a critical role both in diagnosis and in understanding how far the disease has spread. The older approach of full-body X-ray surveys has largely been replaced because X-rays miss early bone involvement in up to 40% of cases, particularly in the spine and pelvis.
Low-dose whole-body CT is now the standard first-line imaging tool for detecting bone destruction. It picks up smaller lesions than plain X-rays and takes only a few minutes. MRI is even more sensitive, particularly for spotting disease inside the bone marrow before it has caused visible holes. In one comparative study, MRI upstaged 11 out of 41 patients compared with CT, meaning it found more extensive disease than CT detected. PET-CT, which combines a CT scan with a radioactive tracer that highlights metabolically active cells, is the preferred choice when doctors suspect myeloma has spread outside the bones into soft tissue.
Genetic Testing for Risk Stratification
Once myeloma is confirmed, a portion of the bone marrow sample is sent for genetic analysis using a technique called FISH (fluorescence in situ hybridization). This test looks for specific chromosomal abnormalities in the myeloma cells that predict how aggressively the disease is likely to behave.
Certain genetic changes are classified as high risk. Deletion of a region on chromosome 17 (where a key tumor-suppressing gene sits) is considered high risk across all major guidelines. Translocations where parts of chromosomes 4, 14, 16, or 20 swap places, especially combined with extra copies of a segment on chromosome 1, also fall into the high-risk category. Standard-risk genetic profiles include hyperdiploidy (extra whole chromosomes) and certain other translocations that tend to respond better to treatment and progress more slowly.
This genetic information doesn’t change whether you have myeloma, but it directly shapes treatment decisions and helps set realistic expectations for how the disease may respond.
How Staging Works
After diagnosis, myeloma is staged using the Revised International Staging System, which combines blood markers with genetic findings to sort patients into three risk groups.
Two blood proteins drive the staging. Beta-2 microglobulin is a small protein shed by myeloma cells; higher levels indicate more disease. Serum albumin, a protein made by the liver, tends to drop as myeloma burden increases. Stage I requires beta-2 microglobulin below 3.5 mg/L and albumin above 3.5 g/dL, along with standard-risk genetics and normal LDH (an enzyme that rises when cells are turning over rapidly). Stage III applies when beta-2 microglobulin exceeds 5.5 mg/L and either high-risk genetic changes or elevated LDH are present. Stage II covers everyone in between.
Staging provides a general prognosis but doesn’t dictate a rigid treatment path. Two people with the same stage can have very different outcomes depending on their overall health, genetic subtype, and how well they respond to initial therapy.
What the Process Looks Like in Practice
For most people, the diagnostic journey begins with routine blood work that flags something unexpected: an abnormal protein, low blood counts, or elevated calcium. From there, the workup expands quickly. You can expect multiple blood draws, a 24-hour urine collection, and imaging within the first few days. The bone marrow biopsy is usually scheduled within a week or two of the initial findings.
Basic biopsy results typically come back within a few days. FISH genetic testing and flow cytometry (which characterizes the surface markers on your plasma cells) can take one to two weeks. Once all results are in, your hematologist can confirm the diagnosis, assign a stage, and begin discussing treatment options. The full process from first abnormal lab result to a complete diagnosis with staging generally spans two to four weeks.