AFB-detection by smear examination ZN Stain Sputum, 1 samples

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Frontline diagnostic method for pulmonary tuberculosis (TB)

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AFB-Detection by Smear Examination ZN Stain Sputum - Comprehensive Guide

Why is it done?
- Detection of Mycobacterium tuberculosis: This test identifies acid-fast bacilli (AFB), primarily Mycobacterium tuberculosis, which are the causative organisms of tuberculosis (TB)
- Diagnostic confirmation: Used to confirm suspected TB in patients presenting with symptoms such as persistent cough, hemoptysis, fever, night sweats, and weight loss lasting more than 2-3 weeks
- Assessment of infectivity: Determines whether a TB patient is infectious (smear-positive) or non-infectious (smear-negative) and helps assess the severity of disease
- Treatment monitoring: Tracks response to anti-tuberculosis therapy by comparing AFB counts at baseline and during treatment (typically at 2 weeks, 2 months, and 5 months)
- Screening of high-risk populations: Routinely performed in individuals with HIV/AIDS, immunocompromised states, close TB contacts, and healthcare workers
- Timing: Initial diagnosis typically uses early morning sputum samples collected on 3 consecutive days; treatment monitoring uses samples at predetermined intervals
Normal Range
- Negative result: No AFB observed under microscopy (Normal/Reference value). Reported as 'Negative' or 'AFB Not Seen'
- Reporting scale for positive results: WHO standardized grading system: • Scanty: 1-9 AFB per 100 oil immersion fields • 1+: 10-99 AFB per 100 oil immersion fields • 2+: 1-10 AFB per oil immersion field • 3+: >10 AFB per oil immersion field
- Units of measurement: AFB count per oil immersion field (x1000 magnification) or per 100 fields as specified by laboratory protocol
- Interpretation significance: Negative = Normal (no TB detected); Any positive grade = Abnormal (TB likely present and potentially infectious)
- Quality indicators: Specimens must contain pus or mucoid material; watery, saliva-only samples may be rejected as inadequate
Interpretation
- Negative result: No acid-fast bacilli detected. Patient is likely not infectious by respiratory route. Does not completely rule out TB (especially in early disease or endobronchial TB), and further investigation with culture or GeneXpert MTB/RIF may be warranted if clinical suspicion remains high
- Scanty positive: 1-9 AFB per 100 fields. Indicates low bacillary load; patient has TB and is potentially infectious but with lower infectivity risk. Requires confirmation and treatment initiation
- 1+ positive: 10-99 AFB per 100 fields. Moderate bacillary load; patient is smear-positive TB case with moderate infectivity
- 2+ positive: 1-10 AFB per field. High bacillary load; patient is highly infectious and contagious
- 3+ positive: >10 AFB per field. Very high bacillary load; patient is maximally infectious; indicates severe pulmonary TB with extensive cavitary disease likely
- Factors affecting interpretation: • Sample quality: Inadequate samples (saliva only) lead to false negatives • Timing: Early morning samples have higher yield • Staining technique: Improper ZN staining affects accuracy • Specimen handling: Improper storage/transport decreases sensitivity • Patient factors: Immunosuppression may reduce AFB shedding
- Clinical significance: Smear-positive patients are epidemiologically important as they represent the main source of TB transmission in the community. Treatment response is assessed by conversion from positive to negative, typically expected by 2 months in drug-susceptible cases
- Limitations: Sensitivity is 40-80% depending on disease stage; cannot differentiate TB from non-tuberculous mycobacteria; does not provide drug susceptibility information
Associated Organs
- Primary organ system: Respiratory system (lungs, bronchi, trachea); sputum samples originate from lower respiratory tract
- Pulmonary tuberculosis: Primary manifestation of TB; accounts for 80-85% of TB cases; directly detected by this test
- Associated conditions and complications: • Cavitary TB: Advanced disease with lung cavitation and higher infectivity • Endobronchial TB: Involves the bronchial tree; may present with negative smears initially • TB with hemoptysis: Indicates advanced cavitary disease or vessel erosion • Fibrocaseous TB: Chronic form with fibrosis and necrosis • Multi-drug resistant (MDR-TB): Resistant strain requiring extended treatment
- Extrapulmonary involvement: TB can disseminate to lymph nodes, pleura, bones, joints, central nervous system, kidneys, and adrenal glands; this test specifically detects pulmonary/endobronchial disease
- Potential complications with positive results: • Respiratory distress from extensive lung involvement • Massive hemoptysis from vessel erosion • Pneumothorax from rupture of subpleural cavities • Acute respiratory distress syndrome (ARDS) • Respiratory failure requiring mechanical ventilation • Secondary bacterial infections • Systemic dissemination (miliary TB) if untreated
- Systemic effects: Positive AFB results indicate active TB infection with potential for constitutional symptoms (fever, chills, night sweats) and immune system activation
Follow-up Tests
- Culture and drug susceptibility testing (DST): Gold standard for TB diagnosis; determines drug susceptibility patterns; recommended for all smear-positive cases to guide therapy and identify MDR-TB or XDR-TB
- GeneXpert MTB/RIF: WHO-recommended rapid molecular test; detects TB and rifampicin resistance within 2 hours; has higher sensitivity than smear microscopy, especially in smear-negative patients
- Chest X-ray: Imaging confirmation showing characteristic findings (upper lobe infiltrates, cavitation); helps assess disease extent and identifies complications
- HIV testing: Mandatory for all TB patients; HIV co-infection affects disease presentation, treatment, and prognosis; influences prophylaxis recommendations
- Repeat sputum smears for treatment monitoring: Collected at 2 weeks, 2 months, and 5 months during treatment; conversion from positive to negative indicates treatment success
- Liver and kidney function tests: Baseline assessment before anti-TB drug initiation; monitoring during therapy to detect drug-induced hepatotoxicity
- CD4 count and viral load (if HIV-positive): Determines need for ART initiation; affects TB treatment regimen and timing
- Tuberculin skin test (TST) or interferon-gamma release assay (IGRA): For contacts and screening; less useful in active disease but helpful for latent TB identification in high-risk populations
- Contact tracing and preventive therapy: Close contacts of smear-positive patients should be identified and tested; latent TB infection in contacts may require preventive therapy
Fasting Required?
- Fasting requirement: NO - Fasting is NOT required for AFB smear examination of sputum samples
- Patient preparation and instructions: • Collect early morning sputum samples (6-8 AM preferred) as bacillary load is highest • Rinse mouth with water (not mouthwash) before collection to reduce contamination with oral flora • Do NOT use toothpaste or antiseptic mouthwashes before collection • Take deep breath and expel sputum from deep respiratory tract (not saliva) • Collect into sterile, labeled, leak-proof containers • Minimum 5 mL of sputum required for adequate examination
- Dietary considerations: Patient may eat or drink normally; no dietary restrictions apply to sputum collection
- Medications: Continue all regular medications as scheduled; no medications need to be withheld or avoided before sputum collection
- Additional special instructions: • Avoid eating/drinking 15 minutes before collection to improve sputum quality • Avoid smoking before collection if possible • If unable to produce sputum naturally, inhalation of normal saline aerosol may help • For tuberculosis suspects, ideally collect 3 samples on 3 consecutive days for initial diagnosis • For treatment monitoring, collect on the same day each week if possible • Transport samples to laboratory within 3 hours or refrigerate at 2-8°C if delayed transport
- Safety precautions: Patients being tested should cover mouth/nose when coughing; healthcare workers handling samples must use appropriate PPE including respiratory protection; specimens should be handled as biohazardous materials

How our test process works!