Sample rejection rate is an important parameter used in the quality monitoring of medical laboratories. We investigated the effect of the COVID-19 pandemic on the monthly rejected sample analysis used as a quality indicator in the preanalytical process in our study. Monthly rejected sample analyzes were divided into two groups as six months before the pandemic (September 2019- February 2020) and six months during the pandemic (April 2020-September 2020). Rejection rates were determined retrospectively by dividing the monthly number of rejected samples to the total number of accepted samples obtained retrospectively via the laboratory information system. Six sigma values and quality indicator error rates were calculated. Pre-pandemic and pandemic data were compared with the paired t-test. The reasons for rejection were evaluated according to the frequency with the Pareto chart. The distribution of rejected samples according to the laboratory department and the medical units where the specimens were sent was examined. Compared to the pre-pandemic period, the rate of insufficient volume of samples increased statistically significantly during the pandemic, while incorrect requests and the other reasons for rejection decreased significantly. A significant increase was observed in the rejected coagulation samples. The rate of rejected samples from intensive care units increased during the pandemic. The COVID-19 pandemic affected the rejected sample analysis. In order to reduce the preanalytical errors, corrective and preventive actions should be taken also during the pandemic.