What labs and interventions are recommended for suspected infections?

Laboratory Tests and Interventions for Suspected Infections

For patients with suspected infections, a complete blood cell count with differential should be performed as the primary initial laboratory test, with additional targeted testing based on the suspected site of infection. 1

Initial Laboratory Evaluation

Complete Blood Cell Count (CBC)

• A CBC with differential should be performed for all patients suspected of having an infection within 12-24 hours of symptom onset 1
• Key findings that strongly suggest bacterial infection:
  • Elevated WBC count >14,000 cells/mm³ (likelihood ratio 3.7) 1, 2
  • Left shift (bands >16% or total band count >1,500 cells/mm³) (likelihood ratio 7.5-14.5) 1, 2
  • High percentage of neutrophils (>90%) (likelihood ratio 4.7-7.5) 1, 2

Urinalysis and Urine Culture

• Do not perform urinalysis or urine cultures for asymptomatic patients 1, 2
• Only test non-catheterized patients with acute onset of UTI symptoms:
  • Fever, dysuria, gross hematuria, new/worsening urinary incontinence 1
• For catheterized patients, evaluate only if suspected urosepsis, especially after recent catheter obstruction or change 1
• Minimum urinalysis should include:
  • Dipstick for leukocyte esterase and nitrite 1
  • Microscopic examination for WBCs 1
  • Only order urine culture if pyuria is present (≥10 WBCs/high-power field or positive leukocyte esterase/nitrite) 1

Site-Specific Evaluations

Respiratory Infections

If pneumonia is clinically suspected:

1. Perform pulse oximetry for patients with respiratory rates >25 breaths/minute 1
2. Order chest radiograph if hypoxemia is documented (oxygen saturation ≤90%) 1
3. Collect respiratory secretions (sputum) to assess for purulence 1
   • Submit purulent sputum for Gram stain and culture if specimen can be transported within 1-2 hours 1

Bloodstream Infections

• Blood cultures have low yield in most long-term care facility residents and rarely influence therapy 1
• Consider blood cultures only when:
  • Bacteremia is highly suspected 1
  • Quick access to laboratory facilities is available 1
  • Adequate physician coverage to respond to results exists 1
  • Capacity to administer parenteral antibiotics is present 1
• When indicated, collect 2-4 blood cultures before starting antibiotics 1, 3
  • Two blood cultures detect approximately 90% of bloodstream infections 3
  • Three blood cultures detect 96-98% of bloodstream infections 3

Central Nervous System Infections

For suspected meningitis:

• Collect specimens prior to initiating antimicrobial therapy 1
• Obtain 2-4 blood cultures 1
• Collect as much CSF as possible (minimum 1 mL) 1
• Do not refrigerate CSF specimens 1

Common Pitfalls to Avoid

1. Ordering "routine" laboratory panels without specific clinical indications 2
2. Ignoring abnormal results in the absence of fever, as many elderly patients with infections don't present with fever 2
3. Overreliance on normal WBC counts to rule out bacterial infection - while elevated counts strongly suggest infection, normal counts don't exclude it 4, 5
4. Testing asymptomatic bacteriuria - present in 15-50% of non-catheterized LTCF residents and nearly 100% in long-term catheterized residents 1, 2
5. Delaying antimicrobial therapy while waiting for culture results in critically ill patients 1

Documentation Requirements

• Document the full extent of clinical evaluation in the medical record 1
• If specific diagnostic measures are consciously withheld, record the reasons 1
• Relay all abnormal laboratory findings to the responsible clinician (physician, nurse practitioner, or physician assistant) 1, 2

Remember that laboratory tests should only be performed if they have reasonable diagnostic yield, are low risk, reasonable in cost, and will improve patient management. Additional testing should be considered only when clinical presentations are unusual, initial therapy fails, or prolonged antimicrobial therapy is being considered.