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Aside from testing to see whether a patient has COVID-19, there are a few tests that may be useful in COVID-19 in order to help us identify who are the sickest patients. Swabs and antibody tests can be useful in determining whether someone has, or has had, the disease but neither of them can give us an idea of disease severity.

One of the difficulties in managing patients with COVID-19 is that it is such a new disease. We’re learning about it all the time. The more experienced doctors get with the disease the more we can see different patterns emerge. Having more information on which patients are in the highest risk groups means that treatments can be allocated more effectively.

One such pattern that has been observed by doctors treating patients with COVID-19 has been in a type of white blood cell called lymphocytes. In an article published in the prestigious Nature journal, researchers described how they observed low levels of blood lymphocyte percentage between days 10 and 12 of the illness. They found that at this time point patients with a low lymphocyte count but which was above 20% had moderate disease and recovered quickly. Those with lymphocyte percentage less than this has a more severe illness.

At a second later time point, at days 17-19 of symptom onset, patients with lymphocyte percentage of less than 5% tended to be very ill, those with values of less than 20% were still ill and needed monitoring and those values of greater than 20% were in the recovery phase.

It’s not clear why COVID-19 might cause low levels of lymphocytes but there are 4 possibilities. The first is that the virus binds to the ACE2 receptor which is present on lymphocytes and destroys them directly. The second is that the virus damages organs that produce lymphocytes. A third possibility is that the inflammatory response to the virus by the body may cause a reduction in the level of lymphocytes. Finally, it may be that an adverse metabolic process that the virus causes in the body may reduce the level of lymphocytes.

Another blood marker that has been used to evaluate disease severity and prognosis in patients with COVID-19 is c-reactive protein or CRP. This is a protein that is made in the liver and is a marker of inflammation in the blood. The production of CRP is triggered by interleukin-6 (IL-6), a pro-inflammatory signalling protein that is the target of one potential treatment for severe COVID-19.  In a study published in the Journal of the American Medical Association, Young and his colleagues found low CRP levels in COVID-19 patients not requiring oxygen (on average 11) and high CRP levels in those COVID-19 patients who did require oxygen (on average 66). Another journal article published based on the experience of COVID-19 in Wuhan demonstrated that patients with a very high CRP level had a much higher risk of dying.

A study done in Beijing examined what was the strongest predictor of severe illness and the risk of dying in patients with COVID-19. They found the having a low level of lymphocytes in combination with a high level of neutrophils was highly predictive of severe illness and death. Neutrophils are a type of white blood cell that increases quickly when tissue in the body is damaged or injured. The researchers found that the risk of severe illness was particularly high in patients aged over 50 who had a ratio of neutrophils to lymphocytes of 3.13.