The Diagnostic Window
Direct answer: The diagnostic window is the period after infection when a given marker is detectable — antigen early, antibody later, and PCR earliest.
The period after infection during which a given test can detect it — and why choosing the wrong marker misses cases.
| Marker | Detectable from | Indicates |
|---|---|---|
| Antigen | Early (acute phase) | Active pathogen present |
| IgM antibody | ~days after onset | Recent infection |
| IgG antibody | ~weeks after onset | Past infection / immunity |
| Nucleic acid (PCR) | Very early | Active infection (gold standard) |
Practical rule
Test too early and the antibody may not have appeared; test too late and the antigen may be gone. Combined antigen+antibody devices widen the window. See Dengue NS1 vs IgM/IgG for a worked example.
By Progene Bio Regulatory Affairs Team · Medical Device QA · Medical disclaimer · Last reviewed: 2026-09-12