Differential Diagnosis of COVID-19: Importance of Measuring Blood Lymphocytes, Serum Electrolytes, and Olfactory and Taste Functions.

Coronavirus disease 2019 (COVID-19) is associated with various symptoms and changes in hematological and biochemical variables. However, clinical features, which can differentiate COVID-19 from non-COVID-19, are not clear. We therefore examined the key clinical features of COVID-19 and non-COVID-19 patients. This study included 60 COVID-19 patients and 100 non-COVID-19 patients, diagnosed by PCR, and no significant differences in the age and sex were seen between the two groups. The frequencies of fatigue, loose stool, diarrhea, nasal obstruction, olfactory dysfunction, taste dysfunction, underlying hyperlipidemia, and the prescription of angiotensin II receptor blocker (ARB) were significantly higher in COVID-19 patients than those in non-COVID-19 patients. The counts of leucocytes, neutrophils, lymphocytes, eosinophils, monocytes, and basophils and the levels of chloride and calcium in blood of COVID-19 patients were significantly lower than those of non-COVID-19 patients. The frequencies of atypical lymphocytes and the levels of lactate dehydrogenase (LDH) and potassium were significantly higher in COVID-19 than those in non-COVID-19. The C-reactive protein (CRP) level in COVID-19 patients was significantly lower than that in non-COVID-19 patients, when we compared CRP levels among patients with elevated CRP. This study is the first to indicate that electrolyte levels and the frequency of atypical lymphocytes in COVID-19 are significantly different from those in non-COVID-19. Fatigue, loose stool, diarrhea, nasal obstruction, olfactory dysfunction, and taste dysfunction were the key symptoms of COVID-19. Furthermore, hyperlipidemia and ARB may be risk factors of COVID-19. In conclusion, leucocytes, leucocyte fractions, CRP, LDH, and electrolytes are useful indicators for COVID-19 diagnosis.