A clinician's guide to infectious diseases diagnostics: principles, pitfalls, and stewardship

Accurate interpretation of infectious diseases diagnostic tests requires a comprehensive understanding of test principles, performance characteristics, and the cognitive discipline of diagnostic stewardship. While contemporary modalities range from traditional culture to high-throughput metagenomics, postgraduate trainees may struggle to reconcile complex laboratory data with the clinical context. This educational primer aims to bridge the gap between laboratory science and bedside decision-making. We first distinguish between analytical (limit of detection) and clinical sensitivity, emphasizing how disease prevalence dictates predictive values. We then move beyond theoretical limitations to identify clinician-modifiable pre-analytical variables, such as specimen collection techniques, that directly affect diagnostic test performance. A core framework for Diagnostic Stewardship is introduced, focusing on the test-treatment threshold and the critical distinction between organism detection and active infection. Through illustrative case studies, we apply clinical reasoning strategies to resolve common pitfalls, including serological cross-reactivity, polymerase chain reaction inhibition in critical samples, and prozone phenomena. Enhanced diagnostic competence requires more than memorizing test characteristics; it demands the integration of stewardship principles, awareness of cognitive biases, and active collaboration with other healthcare professionals. This review provides a structured approach for trainees to navigate the complexities of modern infectious disease diagnostics. Key messages Infectious disease tests should be interpreted in light of the syndrome, pre-test probability and the timing, and type of specimen tested. NAAT positivity detects nucleic acid, not necessarily viable organisms or active disease. Most avoidable microbiological diagnostic errors begin before the sample reaches the laboratory. Discordant test results should trigger a deliberate diagnostic pause and, when needed, discussion with the laboratory.