Diagnostic Testing and Point-of-Care Interpretation
Pre-test probability drives interpretation
Point-of-care testing is only useful when it changes management. Understand sensitivity (few false negatives), specificity (few false positives), and how prevalence changes positive predictive value. A highly sensitive test in a low-prevalence population generates false positives; a negative result is most reassuring when sensitivity is high.
Know the common waived tests: rapid strep, rapid influenza, urine dipstick and pregnancy test, glucose, hemoglobin, lipid panels, PT/INR, HIV antibody, and some molecular assays. Understand CLIA waiver implications and quality control. Interpret urine dipstick with caution: leukocyte esterase and nitrite suggest UTI, but contamination, asymptomatic bacteriuria, and vaginal discharge confound results; microscopy is not waived. Rapid strep sensitivity is roughly 85–90%, so a negative test in a child with high pre-test probability may warrant culture or treatment per guidelines.
ECG interpretation in urgent care: identify ST elevation, ischemia, arrhythmia, prolonged QT, and bundle branch block; do not use a single troponin to exclude acute coronary syndrome. Point-of-care ultrasound can assess pregnancy, gallbladder, bladder volume, and cardiac activity within scope. Pediatric and geriatric reference ranges differ; hemolysis and lipemia distort chemistry results.
Traps: treating a positive test without considering prevalence; ignoring a normal test in a high-risk patient; misreading urine contamination as infection; and using point-of-care troponin as a rule-out.
Worked example: a 6-year-old with fever, exudative pharyngitis, and no cough has a negative rapid strep test. Given high pre-test probability, guidelines support backup culture or empiric treatment depending on local resistance and policy. Rule of thumb: order a test only when the result will change what you do.