Q: What is a safe lymphocyte count?

What is a safe lymphocyte count?

There is no definitive answer and it depends on the context. For example, the older you are the more senescent your immune system is the more likely you are have immunological problems with higher lymphocyte counts.

In general, I consider a total lymphocyte count of 800/mm<sup>3</sup> or higher to be safe. This based on data from the use of other immunosuppressive therapies, such as azathioprine. If you have a count above 800/mm<sup>3</sup> you are very unlikely to get opportunistic infections. Saying this even if you lymphocyte count is lower than 800/mm<sup>3</sup> doesn't mean you are not capable of fighting infections, such as COVID-19. It is also important to remember that less than 2% of your total lymphocyte pool circulates in your blood, therefore, there are many more lymphocytes in your secondary lymphoid organs (lymph nodes, spleen, bone marrow) that are capable of mounting an immune response.

Please note that the WHO defines defines a total lymphocyte count of greater than 1000/mm<sup>3</sup> as being normal, which is why so many commentators and other healthcare professionals refer to this cut-off. I use 800/mm<sup>3</sup> because of the link of counts below 800/mm<sup>3</sup> an opportunistic infections.

There is one exception to the above and that is with the S1P modulators (fingolimod, siponimod, ozanimod and ponesimod), which work by trapping lymphocytes in lymph nodes. In general a lymphocyte count above 200/mm<sup>3</sup> is considered to be okay on these agents. The latter cut-off has been set by the European Medicines Agency. In reality, there is no clear correlation between the total lymphocyte count and infections in patients on S1P modulators. Therefore regardless of the lymphocyte count any patient on a S1P modulator must be considered immunosuppressed.

The slide presentation below was prepared for a talk I gave at a recent Trims-online conference. It tries to capture the subtleties of interpreting and managing lymphopaenia in pwMS on DMTs.

Date & Disclaimer: 19-March-2020; please note this information will be time limited and will change as new data emerges.