What are the recommended laboratory tests and treatments for communicable diseases?

Laboratory Testing for Communicable Diseases

The optimal laboratory approach for communicable diseases depends on the specific infection site and suspected pathogen, with specimen collection prior to antimicrobial therapy being critical for diagnostic accuracy. 1

Central Nervous System Infections

Essential Initial Testing

• Collect 2-4 blood cultures before initiating antibiotics if bacterial meningitis is suspected 1
• Obtain a minimum of 1 mL CSF, but ideally 20 mL when possible, with 5-10 mL frozen for additional testing 2
• Never refrigerate CSF specimens—transport at room temperature within 2 hours in closed sterile containers 1, 2

Core CSF Analysis

• Cell count with differential distinguishes bacterial (neutrophil predominance) from viral/aseptic meningitis 2
• Glucose concentration: bacterial meningitis typically shows CSF glucose <35-40 mg/dL or CSF/blood glucose ratio <0.23-0.36 2
• Protein concentration: elevated in bacterial meningitis (typically >220 mg/dL) 2
• Gram stain identifies bacteria in 60-90% of untreated cases, dropping to 40-60% after antibiotic pretreatment 1, 2
• Bacterial culture remains the definitive diagnosis despite potential false negatives after antibiotics 2

Viral Encephalitis/Meningitis Testing

• HSV-1/2 NAAT (PCR) is the test of choice for suspected encephalitis with >95% sensitivity and specificity 1, 2
• Enterovirus NAAT has >95% sensitivity for aseptic meningitis, particularly important in children 1, 2
• VZV NAAT detects varicella-zoster in approximately 60% of meningoencephalitis cases 1, 2
• West Nile virus IgM antibody testing on CSF is preferred over NAAT, which has <60% sensitivity in immunocompetent hosts 1
• Parechovirus NAAT should be ordered for young infants with sepsis-like presentations 2

Immunocompromised Host Testing

• CMV NAAT on CSF has 82-100% sensitivity and 86-100% specificity for CNS CMV infection in HIV patients 1, 2
• Quantitative EBV NAAT helps distinguish true infection from latent virus 1
• HHV-6 NAAT on CSF for human herpesvirus-6 encephalitis 1, 2
• JC virus NAAT for progressive multifocal leukoencephalopathy 1, 2
• Cryptococcal antigen testing has replaced India ink stain with >90% sensitivity and specificity 1, 2

Chronic Meningitis Evaluation

• Large CSF volumes (≥5 mL) significantly increase sensitivity for mycobacterial and fungal detection 1, 2
• Both NAAT and culture should be requested for M. tuberculosis, as NAAT sensitivity in nonrespiratory specimens may be poor 1
• Complement fixation on CSF is the optimal test for coccidioidal meningitis, as direct smear and culture are often negative 1

Ocular Infections

Specimen Collection Principles

• Label specimens with specific anatomic source (conjunctiva vs. cornea), not just "eye" 1
• Collect dual swabs for conjunctivitis: one for culture and one for Gram stain preparation 1
• Corneal scrapings are preferred over swabs for keratitis diagnosis, as swabs provide minimal material 1
• Obtain paired specimens from the uninfected eye as a control for culture/Gram stain interpretation 1

Diagnostic Testing

• Gram stain is particularly useful for preliminary conjunctivitis diagnosis 1
• Chlamydia trachomatis detection via DFA or NAAT requires appropriate viral transport media 1
• Calcofluor stain for fungi and Acanthamoeba when clinically indicated 1
• Transport specimens on ice immediately for viral culture or NAAT 1

Critical Transport and Timing Considerations

Universal Principles

• Collect all specimens before initiating antimicrobial therapy whenever possible 1, 3
• Room temperature transport within 2 hours is standard for most bacterial and viral specimens 1
• Specimens requiring ice transport include viral cultures, mumps specimens, and rabies samples 1
• Inform the microbiology laboratory when unusual organisms are suspected (Nocardia, fungi, mycobacteria) requiring special procedures 1

Common Pitfalls to Avoid

• Do not rely solely on NAAT for M. tuberculosis in CSF—always request culture as well 1
• West Nile virus IgM may persist >6 months and cause false positives with recent flavivirus vaccination 1
• False positive CSF CMV NAAT results occur in immunocompetent patients with bacterial meningitis 1
• VZV CSF IgM or intrathecal antibody synthesis may be needed to distinguish true meningoencephalitis from post-infectious immune-mediated processes 1
• Toxoplasma serology may be negative in 22% of AIDS patients with toxoplasma encephalitis 1
• Normal skin flora contamination is a concern for ocular specimens—meticulous collection technique is essential 1