Risk factors associated with blood dyscrasia during clozapine treatment: a systematic review and meta-analysis

Clozapine is the only effective treatment for treatment-resistant schizophrenia, but concerns over blood dyscrasia and need for monitoring limit its use. Evidence suggests many hematological abnormalities may result from surveillance bias, with agranulocytosis being the primary adverse effect directly induced by clozapine. This systematic review (PROSPERO: CRD42024487199) investigates non-genetic risk factors associated with blood dyscrasia during clozapine treatment, focusing on neutropenia and agranulocytosis. Random-effect meta-analyses were performed on studies reporting quantitative risk data. Due to inconsistent neutropenia definitions, analyses used absolute neutrophil count (ANC) thresholds of <2000/mm³, <1500/mm³, and <500/mm³. Forty-four studies were included in the systematic review, 15 in meta-analyses. No significant association was found between agranulocytosis and female gender (OR = 1.48, 95% CI: 0.92-2.38; p = 0.106) or age (pooled standardized mean difference [SMD] = 0.32, 95% CI: -0.26 to 0.90, p = 0.285), whilst a modest inverse association with clozapine dose (SMD = -0.32, 95%CI: -0.50 to -0.14, p < 0.001) and baseline white cell count (SMD = -0.21, 95%CI: -0.40 to -0.03, p = 0.026) was found. Neutropenia (ANC < 2000/mm³) was positively associated with concomitant psychotropic use (OR = 2.15, 95% CI: 1.13-4.07, p = 0.019). Clozapine rechallenge studies revealed no significant associations with gender, age, duration of initial clozapine trial, or length of discontinuation period prior to rechallenge. No strong predictors of clozapine-associated blood dyscrasia were identified. Findings may be limited by study variability, surveillance bias, and lack of consistent differentiation between agranulocytosis and milder neutropenia, highlighting limitations in current evidence.