COVID-19 infection causes a reduction in neutrophil counts in patients taking clozapine

BACKGROUND: Monitoring of white cell counts during clozapine treatment leads to cessation of therapy if levels fall below pre-determined values. Reductions in white cell counts, driven by lower levels of lymphocytes, have been observed with coronavirus disease 2019 (COVID-19). Neutropenia during COVID-19 has not been reported. We present data for 56 patients who were taking clozapine and had COVID-19. METHODS: We included patients who were taking clozapine at the time they tested positive for COVID-19. We compared absolute neutrophil counts, lymphocyte counts and white cell counts between baseline and the first week of infection, and baseline and the second week of infection. RESULTS: We observed reductions in absolute neutrophil counts (p = 0.005), lymphocyte counts (p = 0.003) and white cell counts (p < 0.001) between baseline and the first 7 days of COVID-19. All cell counts had returned to baseline levels by days 8 to 14. Six patients experienced neutropenia (absolute neutrophil counts < 2.0 × 10(9)/L) and of those, 4 underwent mandatory cessation of clozapine. For 3 patients, clozapine treatment had been established for more than 6 months with no previous neutropenia, neutrophil levels returned to baseline within 2 weeks and no further neutropenia was observed on restarting treatment. LIMITATIONS: This was a retrospective chart review; larger cohorts are required. Clozapine plasma levels were largely not measured by clinicians. CONCLUSION: These data strongly suggest that mild neutropenia in the acute phase of COVID-19 in patients who are well established on clozapine is more likely to be a consequence of the virus than of clozapine treatment.