Diagnostic Tests: Categories, Characteristics, and Clinical Reasoning

A diagnostic test is any procedure used to identify the presence, absence, or extent of disease. Tests vary widely in methodology, purpose, and performance characteristics.

Major Categories

• Laboratory: Blood (CBC, BMP, LFT, troponin, HbA1c, hormones), urine (UA, culture), CSF, stool, tissue pathology.
• Imaging: X-ray, ultrasound, CT, MRI, PET, nuclear medicine, angiography.
• Endoscopy/Biopsy: Direct visualization + tissue sampling (GI, bronchoscopy, cystoscopy, hysteroscopy).
• Functional/Physiologic: ECG, EEG, EMG/NCS, PFT, stress test, echocardiography.
• Genetic/Molecular: PCR, NGS, FISH, karyotype, biomarker panels.
• Screening: Mammography, colonoscopy, Pap/HPV, PSA, NIPT (applied to asymptomatic populations).

Test Performance Characteristics

• Sensitivity (Sn): True positive rate. High Sn → good for ruling out (SnNout).
• Specificity (Sp): True negative rate. High Sp → good for ruling in (SpPin).
• PPV/NPV: Positive/negative predictive values—depend on disease prevalence.
• Likelihood Ratios (LR+/LR-): Modify pre-test probability to post-test probability; prevalence-independent.

Clinical Reasoning Framework

Diagnostic reasoning follows Bayesian logic:

1. Estimate pre-test probability (history, exam, epidemiology).
2. Select test; apply result.
3. Calculate post-test probability.
4. Compare to decision thresholds (treat, test further, discharge).

Understanding test limitations and pre-test probability is essential to avoid overtesting and misdiagnosis.