Several blood markers have been shown to be associated with severe COVID-19 disease. The level of various biomarkers reflect the pathophysiology of disease and help in assessing the disease severity and thereby play important role in appropriate patient management on right time.
Lymphocyte count: Lymphocyte plays an important role in immune regulation and inflammatory response to protect the body against viral infections. Several meta-analyses have shown the association of lymphopenia with poor disease outcome in hospitalized patients. The possible hypothesis is lymphocyte expresses the receptor ACE-2 and thus attacked and killed by the virus SARS-CoV-2 or increase level of proinflammatory cytokines leads to lymphocyte induced apoptosis. Decrease in lymphocyte count decreases the innate immune power of the host facilitating the progression of disease leading to poor outcome.
C-reactive protein (CRP): This is considered as the most sensitive and reliable biomarker in predicting the severity of COVID-19 disease. This is a non-specific acute phase reactant. It is induced by IL-6 in liver. Elevated CRP level has been observed as a unique feature in COVID-19 unlike other viral infections. The level of CRP is directly related to the level of inflammation and disease severity. It is considered as an early marker. The high early CRP level has been shown to be correlated with lung lesion and reflect the disease severity.
Lactate dehydrogenase (LDH): LDH is found in almost all living cells and catalyzes the interconversion of lactate and pyruvate. Tissue injury releases LDH to the bloodstream, thereby increases LDH level is an indicator of tissue damage. The level of LDH acts as a good early marker of lung injury. High LDH level has been associated with risk of development of severe disease and higher risk of mortality. Several studies also have proposed as prognostic marker of COVID-19.
D-dimer: It is a fibrin degradation product. It is a small protein fragment produced when a blood clot gets degraded by fibrinolysis. Increased inflammatory response and hypoxia due to severe pneumonia lead to activation of coagulation and fibrinolysis leading to elevated D-dimer level. High level of D-dimer has been associated with poor outcome in COVID-19 patients. This is the basis of prophylactic use of anticoagulant in moderate to severe hospitalized COVID-19 patients.
Procalcitonin (PCT): Procalcitonin is a pre cursor of hormone calcitonin. It is produced by several types of cells in the body often in bacterial infections and also due to tissue injury. High level of PCT is one of the established biomarkers for systemic bacterial infection and sepsis. Several meta-analyses have shown association of high level of PCT or constantly rising level with bacterial infection and severe pneumonia or ARDS.
Several other markers like AST, ALT, CK and creatinine have also been associated with severe COVID-19 disease.
