A diagnostic measure called plasma cell labeling index (PCLI) can help predict the likelihood that someone with smoldering multiple myeloma will eventually get multiple myeloma, wrote researchers in a letter to the editor published in this month’s Mayo Clinic Proceedings.

People with smoldering multiple myeloma have abnormal levels of certain cells and proteins in their blood, but don’t feel the symptoms of multiple myeloma. They are at risk for getting the cancer later.

The amount of plasma cells in a patient’s bone marrow and the amount of M protein in a patient’s blood, or serum, are already used in determining prognoses for people with smoldering myeloma. However, PCLI may provide a finer-tuned prognosis, especially for people with a medium-risk prognosis according to their bone marrow plasma cell and M protein amounts.

“The PCLI measures the proliferative rate of the malignant bone marrow plasma cells,” wrote lead investigator Dr. Madan in an email to The Beacon. “It provides a useful measure of how active the disease is and how it will behave in the near future. It can show a change before the M-protein or percent plasma cell changes.”

Those with a PCLI greater than one percent have a poorer prognosis than those who have a PCLI less than one percent. In their letter to the editor, Dr. Madan and his colleagues at the Mayo Clinic wrote about a study they conducted that shows that PCLI may work in the same way in smoldering myeloma prognoses, too.

The researchers looked at 161 people who were diagnosed with smoldering multiple myeloma and received a plasma cell labeling test. The scientists divided their patients into three groups based on the patients’ bone marrow plasma cell and M protein amounts. For these two measures, greater numbers correspond to poorer prognoses.

The researchers found that for patients with more than 10 percent bone marrow plasma cells but less than 3 g/dL serum M protein, 83 patients out of the 161 studied, having a PCLI greater than one was associated with progressing to multiple myeloma. Two years after their smoldering myeloma was diagnosed, 75 percent of patients with PCLI greater than one progressed to multiple myeloma, compared to 21 percent of patients with PCLI less than one. After five years, all of the patients with PCLI greater than one had multiple myeloma, compared to less than half of patients with PCLI less than one.

For people with both bone marrow plasma cells greater than 10 percent and serum M protein greater than 3 g/dL—the poorest prognostic numbers—there wasn’t a statistically significant difference between those with a PCLI greater than one and those with a PCLI less than one. Among the least at-risk smoldering myeloma patients, who had less than 10 percent bone marrow plasma cells and greater than 3 g/dL serum M protein, all had PCLI less than one, so researchers were not able to make any comparisons.

PCLI is a useful risk factor for people with smoldering multiple myeloma, concluded the researchers’ letter to the editor. In addition, Dr. Madan wrote that “Smoldering multiple myeloma patients with a PCLI of 1 or greater should be followed more closely with frequent follow up visits and lab tests to ensure the timely institution of effective therapy before end organ damage develops.” Researchers can also use PCLI to design more specific clinical trials.

The main drawback to PCLI is that it isn’t widely available, the researchers added. To explain this, Dr. Madan pointed to a lack of awareness, the fact that multiple myeloma is a rare disease in most institutions, the narrow use of PCLI instruments, and the cumbersome process of bone marrow incubation.

The researchers suggested scientists study other tests that, like PCLI, measure the rate at which cancerous plasma cells are spreading. This rate may be key to yielding more specific diagnoses for people with smoldering multiple myeloma.

For more information, please see the letter to the editor in Mayo Clinic Proceedings ^[1].