Which viral serologies and PCR tests should be ordered for a patient presenting with rash, fever, and sore throat?

Tests for “rash + fever + sore throat”

Order targeted PCR/antigen tests first (not serologies) based on clinical features; add GAS RADT/culture to prevent rheumatic fever risk, and reserve serology for measles/rubella or EBV when PCR is unavailable or late in illness. 1, 2, 3

• Step 1 — Always screen for Group A Strep (morbidity/mortality/QOL):

  • Rapid antigen detection test (RADT) for GAS; if negative in children/adolescents, back-up throat culture; in adults, culture optional. Avoid antistreptococcal antibody titers. 1, 2

• Step 2 — Choose viral PCR/antigen tests by syndrome cues:

  • Classic viral prodrome (cough, coryza, conjunctivitis) with maculopapular rash → test for measles and rubella:
    • Measles rRT-PCR on nasopharyngeal/throat swab and/or urine; if PCR unavailable/late, measles IgM. Rubella rRT-PCR or IgM per local public health protocols. (High public health impact; fastest actionable = PCR.) [Augmented by general practice and surveillance evidence; see supportive data showing PCR improves confirmation in rash illness] 4, 5
  • Exudative pharyngitis, posterior cervical LAD, fatigue, rash (esp. after amoxicillin) → EBV:
    • EBV VCA IgM/IgG ± EBNA (preferred) or EBV PCR if immunocompromised/severe. [General practice supported; EBV common viral pharyngitis cause per IDSA] 1
  • Vesicular or varicelliform rash or painful oral lesions → HSV/VZV:
    • Lesion swab PCR for HSV-1/2 or VZV. If CNS symptoms, CSF PCR for HSV/VZV/enterovirus. 6, 7
  • Hand-foot-mouth or herpangina, summer/fall, ulcers → Enterovirus:
    • Throat/NP or stool PCR for enteroviruses. 6, 7
  • Acute respiratory illness with sore throat during season or outbreak → multiplex respiratory PCR panel (includes influenza A/B, RSV, adenovirus, rhinovirus, common coronaviruses, SARS-CoV-2) using NP swab; prioritize when results change isolation/antivirals and reduce unnecessary antibiotics. 3

• Step 3 — When initial tests negative and rash persists, add targeted second-line tests:

  • Parvovirus B19 (erythema infectiosum pattern, arthralgias, pregnancy, hemolysis) → B19V PCR or IgM. [Supportive evidence of yield in undiagnosed rash/fever cohorts] 8
  • HHV-6/7 (roseola pattern, young children; HHV-7 also in adults) → multiplex PCR if strongly suspected; interpret cautiously due to latency. 9
  • Adenovirus (pharyngoconjunctival fever) → NP/throat PCR. 3

• Pitfalls and caveats (to protect morbidity/mortality/QOL):

  • Do not use antistreptococcal antibody titers for acute pharyngitis diagnosis. 1, 2
  • In adults, routine back-up culture after negative GAS RADT usually unnecessary; in children/adolescents it is required. 1
  • Serologies can lag and cross-react (e.g., flaviviruses); prefer PCR early. Pair samples if using IgM/IgG. 3
  • If encephalitis signs (AMS, focal deficits, seizures), urgent CSF PCR for HSV-1/2, VZV, enterovirus; start empiric acyclovir. 6, 7

• Minimal order sets by presentation:

  • Nonspecific rash + fever + sore throat, no vesicles, no EBV features:
    • GAS RADT ± culture (child/adolescent), respiratory multiplex PCR (includes influenza/RSV/SARS-CoV-2/adenovirus). 1, 3
  • Measles-like triad (cough, coryza, conjunctivitis) with rash:
    • Measles rRT-PCR (NP/throat ± urine) ± measles IgM; add rubella PCR/IgM as indicated; notify public health. 4, 5
  • Mononucleosis-like:
    • EBV VCA IgM/IgG ± EBNA; consider CMV IgM/IgG if EBV negative. 1
  • Vesicular lesions:
    • Lesion PCR for HSV/VZV. 6
  • Hand-foot-mouth/herpangina:
    • Enterovirus PCR (throat/stool). 6

Evidence strength and rationale:

• IDSA 2012 pharyngitis guideline (most authoritative/clinically impactful) directs GAS testing strategy and underscores that clinical features alone are unreliable; avoiding missed GAS reduces rheumatic fever risk. 1, 2
• IDSA/ASM 2018 lab utilization guideline supports NP/throat PCR for respiratory viruses and adenovirus. 3
• ABN/BIA encephalitis guidelines specify HSV/VZV/enterovirus PCR dominance when CNS involvement suspected. 6, 7
• Rash-fever literature supports adding measles PCR/IgM and targeted PCR panels when measles/rubella are in the differential; PCR increases case confirmation. 4, 5
• Broad multiplex panels can improve stewardship and operational efficiency when available. 3