CBC and Urinalysis Findings in Streptococcal Pharyngitis
In uncomplicated streptococcal pharyngitis, neither CBC nor urinalysis is routinely indicated or expected to show specific diagnostic findings, as the diagnosis relies on clinical scoring systems and throat culture/RADT, not laboratory blood or urine tests. 1, 2
Why These Tests Are Not Part of Standard Diagnosis
The IDSA guideline for GAS pharyngitis does not recommend CBC or urinalysis as part of the diagnostic workup for uncomplicated streptococcal pharyngitis 1. The diagnosis is made through:
- Clinical scoring systems (presence of fever, tonsillar exudate, tender cervical lymphadenopathy, absence of cough) 2, 3
- Throat culture or RADT for microbiologic confirmation 1, 4
Expected CBC Findings (When Obtained)
If a CBC is performed for other clinical reasons, you may see:
- Elevated white blood cell count (leukocytosis) - significantly more common in streptococcal infections compared to viral pharyngitis 5
- Elevated C-reactive protein - significantly raised in streptococcal pharyngitis (P = 0.00016) 5
- Nonspecific inflammatory markers - these help differentiate bacterial from viral causes but are not diagnostic for GAS specifically 6, 5
Expected Urinalysis Findings (When Obtained)
In uncomplicated streptococcal pharyngitis, urinalysis should be completely normal. 1
Critical Caveat - When Urinalysis Becomes Important:
Abnormal urinalysis findings indicate poststreptococcal complications and require immediate attention:
- Hematuria and proteinuria suggest poststreptococcal glomerulonephritis, typically occurring 1-3 weeks after pharyngitis in children aged 2-6 years 7, 6
- Elevated creatinine and BUN with urinary abnormalities indicate renal involvement requiring urgent evaluation 7
- These findings warrant checking antistreptolysin O (ASO) titers to confirm recent streptococcal infection 7, 6
Clinical Pitfalls to Avoid
- Do not order CBC or urinalysis routinely for suspected streptococcal pharyngitis - this adds unnecessary cost without improving diagnostic accuracy 1, 2
- Do obtain urinalysis if the patient presents 1-3 weeks after pharyngitis with new symptoms (edema, hypertension, decreased urine output, or hemoptysis) suggesting poststreptococcal complications 7, 6
- Elevated inflammatory markers alone (WBC, CRP) cannot distinguish GAS from other bacterial pharyngitis (groups C, G, F streptococci) - throat culture remains essential 5