What labs should be drawn for a patient presenting with strep throat and flank pain?

Laboratory Tests for Strep Throat with Flank Pain

For a patient presenting with strep throat and flank pain, you should draw throat cultures or rapid antigen detection tests (RADTs) to confirm strep pharyngitis, plus urinalysis, urine culture, complete blood count (CBC), and comprehensive metabolic panel (CMP) to evaluate for possible kidney involvement or complications.

Diagnostic Testing for Strep Pharyngitis

• Throat swab for rapid antigen detection test (RADT) is the initial test of choice for diagnosing strep throat, with excellent specificity (≥95%) 1, 2
• Throat culture should be performed to confirm negative RADT results in children and adolescents, as RADTs have sensitivity of only 80-90% 3, 2
• For adults, negative RADT results may not require culture confirmation due to lower incidence of streptococcal infection and extremely low risk of rheumatic fever 3
• Testing is most appropriate for patients with higher likelihood of streptococcal infection based on Centor criteria (fever, tonsillar exudates, tender cervical lymphadenopathy, absence of cough) 4

Additional Testing for Flank Pain

• Urinalysis should be performed to evaluate for hematuria, pyuria, and proteinuria, which may indicate kidney involvement 1
• Urine culture is necessary to identify potential urinary tract infection that could be causing the flank pain 1
• Complete blood count (CBC) to assess for leukocytosis, which may indicate infection or inflammation 3
• Comprehensive metabolic panel (CMP) to evaluate kidney function (BUN, creatinine) and electrolyte abnormalities 3
• C-reactive protein (CRP) and/or erythrocyte sedimentation rate (ESR) may be indicated if post-streptococcal complications are suspected 3

Evaluating for Complications

• The combination of strep throat with flank pain raises concern for potential post-streptococcal glomerulonephritis, which requires monitoring of kidney function 1
• Anti-DNase B antibody testing may be useful for confirming invasive streptococcal disease but requires serial testing and is not recommended for routine diagnosis 3
• Blood cultures should be considered if there are signs of systemic infection or if the patient appears toxic 4

Common Pitfalls to Avoid

• Failing to consider kidney complications when strep throat presents with flank pain 1
• Relying solely on RADT without culture confirmation in high-risk populations or when clinical suspicion is high despite negative RADT 2
• Treating based solely on antibody test results without considering clinical presentation 1
• Assuming that C-reactive protein levels alone can diagnose acute group A streptococcal sore throat 3

Testing Algorithm

• For all patients with suspected strep throat and flank pain:
  1. Perform throat swab for RADT 3, 2
  2. Collect urinalysis and urine culture 1
  3. Draw blood for CBC and CMP 3
  4. Consider ESR and CRP if post-streptococcal complications are suspected 3
  5. For children and adolescents with negative RADT, confirm with throat culture 3, 2