Principles of molecular and serological testing for infection
How PCR, Ct values, antigen tests and antibody tests work, what each can and can't tell you, window periods, IgM, avidity and paired sera, treponemal versus non-treponemal syphilis tests, hepatitis B patterns, and why low-prevalence screening needs confirmatory testing.
Tier 1Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- Laboratory tests for infection detect the organism (culture), its nucleic acid (PCR and other NAATs), its antigen, or the host antibody response.
- NAATs are sensitive and fast but detect nucleic acid, not live organism.
- Antibodies lag infection by 1 to 3 weeks, so serology is often negative early and is better for diagnosing past exposure or immunity.
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Sources
- Laboratory case definitionsAU/NZPublic Health Laboratory Network (PHLN), Australian Government Department of Health, Disability and Ageing, accessed November 2026
- National HIV Testing PolicyAU/NZASHM on behalf of the Australian Government, online edition on the unified ASHM HIV guidelines site, accessed November 2026
- Australian STI Management Guidelines for Use in Primary Care: syphilisAU/NZASHM, online edition, accessed November 2026
- RCPA ManualAU/NZRoyal College of Pathologists of Australasia, online edition, accessed November 2026
- A guide to utilization of the microbiology laboratory for diagnosis of infectious diseases: 2024 update (IDSA and ASM)Clin Infect Dis 2024;ciae104 (Miller JM et al.), 2024
- Guidelines for diagnosing, preventing and managing cryptococcal disease among adults, adolescents and children living with HIVWorld Health Organization, 2022
- Last updated
- 14 November 2026
What changed
- 14 November 2026 Turnstile bot check on sign-in and signed-out feedback