Measles - PCR - Qualitative

The PCR test for measles detects measles virus RNA in respiratory samples (throat or nasopharyngeal swabs), urine, or blood. It’s highly sensitive and specific for early infection and outbreak confirmation, often returning results within hours to a few days. PCR complements IgM serology and helps confirm diagnosis, guide public health response, and enable viral genotyping to track transmission.

PCR detects measles viral RNA in swabs or urine and is most useful early (before and shortly after rash), giving direct evidence of infection with high sensitivity. IgM is the patient’s antibody measured in blood that appears around the time of rash (often ≥3 days after) and indicates recent infection; it can persist for weeks and may yield false positives/negatives, so timing and clinical context matter.

Measles PCR is most sensitive when throat or nasopharyngeal swabs are taken within 3 days of rash onset, though virus can be detectable in respiratory samples and urine up to about 7 days. Laboratory turnaround typically ranges from 24 to 72 hours, but may take longer if samples are sent to reference labs. For best accuracy, collect specimens early after rash appears.

Confirm measles clinically (high fever, cough, coryza, conjunctivitis, Koplik spots, then a spreading maculopapular rash) and by laboratory tests. Detect measles-specific IgM in serum (appears about 3 days after rash) or measles RNA by RT‑PCR from throat/nasopharyngeal swab or urine. A fourfold rise in measles IgG on paired sera also confirms recent infection.

Measles is confirmed by laboratory testing: molecular detection (RT‑PCR) of measles RNA from throat/nasopharyngeal swab or urine; and serology showing measles‑specific IgM or a significant rise/seroconversion in IgG between acute and convalescent samples. Viral culture is used rarely. Timing matters: PCR is most sensitive early; IgM typically becomes detectable around rash onset.

A PCR (polymerase chain reaction) test detects a pathogen’s genetic material—DNA or RNA—by amplifying specific target gene sequences. For RNA viruses (like SARS-CoV-2) the sample is converted to DNA and amplified; a positive result shows the pathogen’s genetic material is present, indicating likely current infection. PCR is highly sensitive and can detect very low amounts from swabs or other clinical samples.