What is the recommended length of Acid (A) and Phosphatase (P) smear testing for adults at risk for tuberculosis?

AFB Smear Testing for Tuberculosis: Collection Protocol

Collect at least three sputum specimens for acid-fast bacilli (AFB) smear microscopy, with specimens obtained 8-24 hours apart and at least one collected in the early morning. 1

Specimen Collection Requirements

Number and Timing of Specimens

• Three specimens are required as standard practice in the United States, strongly recommended by the CDC and National Tuberculosis Controllers Association 1, 2
• Specimens must be collected 8-24 hours apart to maximize diagnostic yield 1, 2
• At least one early morning specimen is mandatory, as it provides 12% greater sensitivity than spot specimens 1, 3

Rationale for Three Specimens

• The first specimen detects 53.8% of culture-positive TB cases 1, 3
• Adding a second specimen increases sensitivity by 11.1% to approximately 65% 1, 3
• The third specimen adds only 2-5% additional sensitivity but reaches approximately 70% total sensitivity 1, 3
• Three specimens remain standard despite diminishing returns because poor sample quality is pervasive in clinical practice 1, 2

Volume and Processing Requirements

Optimal Specimen Volume

• Request minimum 3 mL sputum volume, but optimal volume is 5-10 mL 1, 2, 3
• Using ≥5.0 mL increases sensitivity to 92% compared to 72.5% with smaller volumes 3

Processing Method

• Concentrated specimens are strongly preferred over direct smears, increasing sensitivity from 34% to 58% for all AFB culture-positive specimens 3
• Fluorescence microscopy is preferred over conventional microscopy, providing 10% greater sensitivity on average 1, 3

Collection Environment and Safety

Infection Control During Collection

• Specimens should be collected in a sputum induction booth or airborne infection isolation (AII) room 1, 2
• In resource-limited settings without environmental containment, collection outdoors is safer 1

Patient Instruction

• Healthcare professionals should supervise and observe sputum collection when possible 1, 2
• Patients must be instructed on producing deep sputum specimens from the lungs, not saliva 1, 2
• For patients unable to produce adequate sputum, induction with hypertonic saline aerosol should be performed 1, 2

Laboratory Reporting Timeline

Turnaround Requirements

• Positive AFB smear results must be reported within 24 hours of specimen collection 1
• Positive cultures must be reported within 24 hours of detection 1, 2
• Laboratories should treat initial positive results as critical values requiring immediate communication to clinicians and public health authorities 1

Critical Limitations to Recognize

Sensitivity Constraints

• Negative AFB smears do NOT exclude TB diagnosis - approximately 40% of culture-confirmed pulmonary TB cases have negative smears even with optimal technique 1, 2, 3
• Only 63% of U.S. patients with culture-positive TB have positive AFB smears 1
• HIV-infected patients are even less likely to have positive smears, consistent with lower rates of cavitary disease 1, 3

Specificity Considerations

• Specificity is ≥90%, meaning positive results are highly reliable 1, 3
• Positive predictive value ranges 70-90% depending on local prevalence of nontuberculous mycobacterial disease 1, 3

Mandatory Concurrent Testing

Culture Requirements

• Culture remains mandatory regardless of smear results - it is the gold standard for TB confirmation and required for drug susceptibility testing 1, 2
• Both liquid and solid mycobacterial cultures should be performed on every specimen 1
• Drug susceptibility testing must be performed on all initial positive cultures 1, 2

Molecular Testing

• Nucleic acid amplification (NAA) testing should be performed on at least one respiratory specimen, preferably the first diagnostic specimen 1, 2
• NAA results should be available within 48 hours of specimen collection 1