What initial laboratory tests should be ordered for a patient with suspected discitis?

Laboratory Evaluation for Discitis

For suspected discitis, obtain erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), complete blood count with differential, and blood cultures before initiating antibiotics. 1, 2

Essential Initial Laboratory Tests

Primary Inflammatory Markers

• ESR is highly sensitive and typically elevated in spine infections, making it the most reliable screening marker for discitis 2
• CRP rises more rapidly than ESR and correlates more closely with clinical status, with levels >100 mg/L indicating very high suspicion for active spinal infection requiring urgent intervention 2
• The combination of elevated ESR and CRP significantly increases diagnostic suspicion for spine infection, and both should be obtained together 2
• WBC count with differential may show leukocytosis but can be normal in up to 40% of patients with spine infection, making it less reliable than ESR and CRP 2

Microbiologic Testing

• Blood cultures must be obtained before starting antibiotics to maximize organism identification, as they are positive in approximately 40-60% of spontaneous discitis cases 1, 3, 4
• Multiple sets of blood cultures should be drawn given the intermittent bacteremia that may occur with discitis 1

Additional Testing Based on Risk Factors

Endemic Infections

• Brucella serology (with cutoff ≥1:160) should be obtained in patients from endemic areas or with unpasteurized dairy exposure, as brucellar discitis is common in endemic regions and serology is highly sensitive 1, 5
• Mycobacterial cultures and interferon-γ release assay should be performed in patients from tuberculosis-endemic areas or with risk factors for TB, as these tests have higher sensitivity than PPD (82.8% vs 58.6%) 1

Immunocompromised Patients

• Fungal cultures and serology are indicated in patients with diabetes, HIV, cancer, or on dialysis who have increased risk for atypical pathogens 2

Laboratory Markers for Treatment Monitoring

• CRP improves more rapidly than ESR during treatment and should be used to monitor therapeutic response 2
• CRP >27.5 mg/L after 4 weeks of treatment indicates treatment failure and significantly higher risk of recurrence 2

Critical Clinical Pitfalls

• Do not delay imaging or treatment while waiting for laboratory results if clinical suspicion is high, as early diagnosis is crucial to prevent neurologic complications 1, 6
• Do not rely solely on WBC count, as normal values do not exclude discitis and ESR/CRP are far more sensitive markers 2
• Obtain blood cultures before antibiotics whenever possible, as prior antibiotic exposure significantly reduces culture yield 3, 4
• Consider CT-guided disc space aspiration if blood cultures are negative, as this increases organism detection from 33% to 67% and allows targeted antibiotic therapy, reducing treatment duration from 142 to 77 days 3

Additional Baseline Testing

• Serum creatinine, electrolytes, and complete metabolic panel should be obtained to assess renal function before initiating potentially nephrotoxic antibiotics and to evaluate for comorbid conditions 1
• Coagulation studies if invasive procedures such as image-guided biopsy are anticipated 1