Confirmatory and repeat testing follows initial screening results to establish or exclude a diagnosis with greater certainty. Confirmatory tests have higher specificity than screening tests, reducing false-positive rates. Repeat testing accounts for pre-analytical variables (specimen handling, patient preparation), biological variation (diurnal variation, acute illness effects), and laboratory error. The concept of test-retest reliability ensures consistent results. Examples include: HIV screening (4th-gen Ag/Ab test) confirmed by HIV-1/2 differentiation assay; diabetes screening (random glucose or A1c) confirmed by repeating on a separate day; CKD diagnosis requiring two abnormal eGFR values ≥3 months apart to distinguish from AKI. Understanding when to repeat vs confirm vs accept results improves diagnostic efficiency and reduces unnecessary testing.