2.5. Reliability of analyses

Possible confounders were analysed. Changes in clozapine dose, the occurrence of fever or inflammation not caused by the vaccination, changes in caffeine or tobacco consumption, and comedication that might affect CYP1A2 activity in the period between baseline and second COVID‐19 vaccination were monitored. The sampling time for blood tests to check clozapine plasma levels was standardized at 12 h (± 1 h) after the last clozapine dose.

Another step was taken to address sources of confounding: the methods for monitoring clozapine blood levels (high performance liquid chromatography [HPLC] or liquid chromatography‐mass spectrometry [LC‐MS]) were determined at baseline, and following the first and second vaccination. The LC‐MS procedure is more sensitive and specific, and gives more accurate results with interassay coefficients of variation (CV) of 3.04%–4.94% for clozapine compared with HPLC, with interassay CVs ranging from 0.99% to 10.14% for clozapine. 45 , 46 After internal analysis of both HPLC and LC‐MS, we found LC‐MS to be more accurate for purification and quantification with an average higher outcome of 22% (Unpublished data analytical method validation Laboratory of Clinical Pharmacy Northwest Hospital; [LC‐MSMS; Recipe TCA calibrators, internal standard and controls; Waters Acquity UPLC^® BEH C18, 1.7 μm, 2.1 × 100 mm; mobile phase A: ammoniumacetate, methanol, aquadest Milli‐Q; mobile phase B: ammoniumacetate, methanol]). Therefore, we corrected all results obtained with HPLC with a factor of 1.22.