Introduction and objectives Common laboratory parameters are crucial in aiding coronavirus disease 2019 (COVID-19) case detection. This study aimed to determine the differences between laboratory parameters in (1) COVID-19 versus non-COVID-19 pneumonia, and (2) severe versus non-severe COVID-19 cases. Methods Studies were collected until March 2020, and retrieved parameters include leukocyte, neutrophil, thrombocyte, and lymphocyte counts in addition to C-reactive protein (CRP), procalcitonin (PCT) and D-dimer levels. In the presence of heterogeneity, the random-effect model (REM) was used instead of the fixed-effect model (FEM). Results Seven studies in the first analysis showed significantly lower leukocyte, neutrophil and platelet counts in COVID-19 pneumonia (SMD = −0.42, 95%CI −0.60 to −0.25, p < 0.00001, SMD = −0.23, 95%CI −0.41 to −0.06, p = 0.01, SMD = −0.54, 95%CI −0.91 to −0.16, p = 0.0005) compared to non-COVID-19 pneumonia. Twenty-six studies in the second analysis showed significantly lower lymphocyte and thrombocyte counts (SMD = −0.56, 95%CI −0.71 to −0.40, p < 0.0001, SMD = −0.32, 95%CI −0.49 to −0.15, p = 0.0002) and significantly higher leukocyte, neutrophil, D-dimer, and CRP (SMD = 0.31, 95%CI 0.07–0.56, p = 0.01; SMD = 0.44, 95%CI 0.24–0.64, p < 0.0001; SMD = 0.53, 95%CI 0.31–0.75, p < 0.00001; SMD = 0.97, 95%CI 0.70–1.24, p < 0.00001) in severe COVID-19 compared to non-severe COVID-19. Conclusions In conclusion, thrombocyte count is key in both diagnosis and prognosis. Low leukocyte and neutrophil counts are markers of COVID-19 infection, but contrastingly higher counts indicate progressive COVID-19. And although lymphocyte, D-dimer and CRP levels did not demonstrate diagnostic value, all indicate severity of COVID-19. Confirmation of these findings should be performed in future studies.