A high leukocyte count and administration of hydrocortisone hamper PCR-based diagnostics for bloodstream infections.

Bloodstream infections (BSIs) require an accurate and fast identification of causative pathogens. Molecular diagnostics, in particular polymerase chain reaction (PCR)-based approaches for BSI diagnostics directly from whole blood, suffer from limitations such as inhibition leading to invalid results. In this retrospective study, we analyzed 23 parameters for their potential interference with LightCycler SeptiFast PCR tests (n = 2167) routinely performed at our institution. The overall inhibition rate was 9.1%. Test date, type of ward, procalcitonin levels, high leukocyte counts, and absolute neutrophil count were significantly associated with inhibition. For a subset (n = 448), cut-off values for leukocyte counts of < 5700 cells/µL and ≥ 26,900 cells/µL were significantly associated with a low (5%) and high (67%) inhibition risk. For patients with a moderate to high leukocyte count (5700-26,900 cells/µL), the additional administration of hydrocortisone significantly increased the inhibition risk. Furthermore, freezing of blood samples prior to DNA extraction and SF testing appeared to neutralize inhibitory factors. It remains to be investigated whether other molecular diagnostic tests are susceptible to similar inhibiting parameters.