Lymphocyte count is a universal predictor of health outcomes in COVID-19 patients before mass vaccination: A meta-analytical study

BACKGROUND: Several laboratory data have been identified as predictors of disease severity or mortality in COVID-19 patients. However, the relative strength of laboratory data for the prediction of health outcomes in COVID-19 patients has not been fully explored. This meta-analytical study aimed to evaluate the prediction capabilities of laboratory data on the prognosis of COVID-19 patients during 2020 while mass vaccination has not started yet. METHODS: Two electronic databases, MEDLINE and EMBASE, from inception to October 10, 2020 were searched. Observational studies of laboratory-confirmed COVID-19 patients with well-defined severity or survival status, and with the desired laboratory data at initial hospital administrations, were selected. Meta-regression analysis with the generalized estimating equations (GEE) method for clustered data was performed sequentially. Primary outcome measures were to compare the level of laboratory data and their impact on different health outcomes (severe vs non-severe, critically severe vs non-critically severe, and dead vs alive). RESULTS: Meta-data of 13 clinical laboratory items at initial hospital presentations were extracted from 76 selected studies with a total of 26 627 COVID-19 patients in 16 countries. After adjusting for the effect of age, 1.03 <lymphocyte count mean or median ( × 10(9)/L) ≤2.06 (estimated odds ratio (OR) = 0.0216; 95% confidence interval (CI) = 0.0041-0.1131; P < 0.0001), higher lymphocyte count mean or median ( × 10(9)/L) (OR <0.0001; 95% CI: <0.0001-0.0386; P = 0.0284), and lymphocyte count mean or median ( × 10(9)/L) >0.87 (OR = 0.0576; 95% CI = 0.0043-0.4726; P = 0.0079) had a much lower risk of severity, critical severity, and mortality from COVID-19, respectively. CONCLUSIONS: Lymphocyte count was the most powerful predictor among the 13 common laboratory variables explored from COVID-19 patients to differentiate disease severity and to predict mortality. Lymphocyte count should be monitored for the prognoses of COVID-19 patients in clinical settings in particular for patients not fully vaccinated.