Which laboratory studies are indicated for evaluating a suspected infection?

Laboratory Studies for Suspected Infection

When evaluating a patient with suspected infection, obtain blood cultures (2-4 sets), complete blood count with differential, inflammatory markers (CRP and/or procalcitonin), and lactate level, along with site-specific cultures guided by clinical presentation. 1

Core Laboratory Studies

Blood Cultures

• Obtain at least 2 sets of blood cultures (aerobic and anaerobic bottles) before starting antimicrobials if this causes no substantial delay (>45 minutes) in antibiotic administration 1
• Draw at least one set percutaneously and one through each vascular access device if the device has been in place >48 hours 1
• Specific indications include: fever, chills, hypothermia, leukocytosis, left-shift neutrophils, neutropenia, hypoalbuminemia, new renal failure, or hemodynamic compromise 1
• Collect blood as soon as possible after fever onset, ideally immediately before the next scheduled antibiotic dose if patient is already on antibiotics 1
• Obtain 20-60 mL total blood volume per culture set (10-30 mL per bottle) using fresh venipuncture with proper skin antisepsis 1
• For suspected bacterial meningitis, obtain 2-4 blood cultures in addition to cerebrospinal fluid studies 1

Complete Blood Count

• Obtain CBC with manual differential within 12-24 hours of symptom onset (sooner if seriously ill) 1
• Leukocytosis (WBC ≥14,000 cells/mm³) or left shift (≥16% bands or ≥1,500 bands/mm³) warrants careful assessment for bacterial infection 1
• Neutropenia is also an important indicator for obtaining blood cultures 1
• WBC and hemoglobin are independent predictors of bacterial infection requiring antibiotics 2

Inflammatory Markers

• C-reactive protein (CRP): CRP ≥50 mg/L has 98.5% sensitivity and 75% specificity for identifying sepsis in ICU patients 1
• Procalcitonin (PCT): PCT ≥1.5 ng/mL has 100% sensitivity and 72% specificity for sepsis 1
• Sequential daily measurements are more valuable than single measurements for diagnosing infection 1
• These markers cannot alone differentiate sepsis from other causes of systemic inflammatory response syndrome (SIRS) but are valuable as part of systematic evaluation 1
• CRP has superior diagnostic accuracy compared to PCT for guiding antibiotic decisions in emergency department patients 2
• Positive PCT correlates with elevated APACHE II scores and negative clinical outcomes 3

Lactate Level

• Obtain serum lactate in patients with suspected sepsis or septic shock 1, 4
• Elevated lactate indicates tissue hypoperfusion and is incorporated into the septic shock definition 4
• Use lactate normalization as a resuscitation target in patients with elevated levels 1

Site-Specific Studies

Urinary Tract Infection

• Perform urinalysis for leukocyte esterase and nitrite by dipstick plus microscopic examination for WBCs 1
• Only order urine culture if pyuria is present (≥10 WBCs/high-power field or positive leukocyte esterase/nitrite) 1
• For suspected urosepsis, obtain paired blood and urine cultures plus Gram stain of uncentrifuged urine 1
• Use midstream clean-catch specimen or in-and-out catheterization; change indwelling catheters before specimen collection in catheterized patients with suspected urosepsis 1

Pneumonia

• Obtain pulse oximetry to document hypoxemia (oxygen saturation <90%) 1
• Chest radiography should be performed if hypoxemia is documented or suspected 1
• Blood cultures accompanying sputum specimens may be helpful, particularly in high-risk community-acquired pneumonia 1
• First morning sputum is optimal for culture 1

Central Nervous System Infection

• Collect cerebrospinal fluid before antimicrobial therapy whenever possible 1
• Attempt to collect maximum volume (minimum 1 mL, ideally ≥5 mL for mycobacterial testing) 1
• Do not refrigerate CSF 1
• Inform laboratory if unusual organisms are suspected (Nocardia, fungi, mycobacteria) requiring special procedures 1
• Gram stain sensitivity is 60-80% in untreated patients, 40-60% in treated patients 1
• Molecular testing (NAAT) has replaced viral culture for enteroviral meningitis and has >95% sensitivity for HSV encephalitis 1

Gastrointestinal Infection

• For symptoms consistent with colitis (severe fever, abdominal cramps, bloody diarrhea), test for Clostridioides difficile initially, especially if recent antibiotic exposure 1
• For small bowel symptoms persisting >7 days, examine stool for Giardia and other protozoa 1

Important Caveats

Timing Considerations

• Antibiotics, bismuth, and proton pump inhibitors decrease bacterial load and can cause false-negative results in blood cultures, rapid urease tests, histology, urea breath tests, and stool antigen tests 1
• Discontinue these agents for at least 2 weeks before testing when possible 1
• However, a positive result can be trusted despite these medications 1
• Histamine-2 receptor antagonists do not affect bacterial load and can substitute for PPIs 1

Special Populations

• Long-term care facility residents: Blood cultures have low yield and rarely influence therapy; not recommended for most residents unless bacteremia is highly suspected with quick laboratory access 1
• In the absence of fever, leukocytosis, left shift, or focal infection signs, additional diagnostic tests may have low yield 1

Specimen Collection Technique

• Use proper skin antisepsis with 70% isopropyl alcohol or iodine-containing solution applied twice 1
• Avoid sites with skin contamination (femoral) or loss of skin integrity 1
• Change needle before inoculating culture bottles 1
• Use media containing antibiotic-adsorbing substances (BacT/Alert FAN, BACTEC Plus/F) for patients already on antibiotics 1