Diagnostic methods for tuberculosis and their applicability in Indonesia (2019) N004

Overview of Tuberculosis (TB) Diagnostic Methods

• Tuberculin Skin Test (TST):

  • The TST is a delayed-type hypersensitivity reaction used to detect TB infection.

  • Limitations: Includes cross-reactivity with other antigens, low sensitivity and specificity, and false-positive results—especially in individuals vaccinated with BCG.

  • Procedure: Involves intradermal injection of purified protein derivative (PPD).

  • Interpretation:

    • ≥5 mm induration: positive in immunocompromised patients.

    • ≥10 mm: positive in immunocompetent children and unvaccinated adults.

    • ≥15 mm: positive in vaccinated individuals.

  • Accuracy:

    • Sensitivity/Specificity:

      • Immunocompromised: 100% / 90.3%

      • Immunocompetent: 97.2% / 91.9%

      • Vaccinated: 86.1% / 94.2%

  • Cross-reactivity with BCG and environmental mycobacteria may yield up to 86.1% false positives in vaccinated persons.

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• Interferon-Gamma Release Assay (IGRA):

  • Measures the interferon-gamma (IFN-γ) response to TB-specific antigens such as ESAT-6, CFP-10, and TB7.7.

  • Uses ELISA (e.g., QuantiFERON®-TB Gold In-Tube) or ELISpot (e.g., T-SPOT®.TB™) techniques.

  • Advantages:

    • Higher specificity than TST.

    • Not affected by BCG vaccination.

    • Especially useful for diagnosing TB in immunocompromised and TB-HIV co-infected patients.

  • Clinical Utility: Preferred for screening latent TB in patients with inflammatory diseases or compromised immunity.

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• GeneXpert MTB/RIF Assay:

  • A cartridge-based nucleic acid amplification test (NAAT) using quantitative real-time PCR (qRT-PCR).

  • Detects M. tuberculosis DNA and rifampicin resistance within 2 hours.

  • Key Features:

    • Closed amplification system minimizes cross-contamination.

    • Detects mutations in the rpoB gene, responsible for ~96% of rifampicin resistance.

    • Identifies bacteria in both positive and negative smears.

    • Can process multiple specimen types (sputum, CSF, pleural fluid, etc.).

  • Limitations:

    • False positives: due to target amplification errors.

    • False negatives: due to poor sample quality, PCR inhibitors, or reagent issues.

    • Requires stable electricity, temperature below 30°C, and regular equipment calibration.

    • Uses Bacillus globigii spores as internal controls for quality assurance.

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Transmission Factors of Tuberculosis

• Infectivity and Disease Severity:

  • Determined by sputum smear results and chest X-rays.

• Contact History and Duration:

  • Prolonged and frequent exposure increases infection risk.

• Environmental Conditions:

  • Transmission is higher in enclosed, poorly ventilated spaces.

• Strain Virulence:

  • Some M. tuberculosis strains are more infectious; extrapulmonary strains tend to cause greater macrophage damage than pulmonary strains.

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Diagnostic Indicators and Methods

• Clinical and Radiological Findings:

  • Positive acid-fast bacillus (AFB) smear or abnormal chest X-ray supports TB infection.

  • Radiographic signs include hilar lymphadenopathy, consolidation, pleural effusion, miliary lesions, atelectasis, calcifications, and tuberculoma.

• Diagnostic Workflow:

  1. AFB staining (Ziehl-Neelsen, Kinyoun–Gabbet, or Auramine-Rhodamine).

     • Fast and inexpensive; sensitivity ≈70%.

     • Operator-dependent; negative results may require further culture testing.

  2. Culture method – the gold standard.

     • Specimens: sputum, urine, CSF, pleural fluid, pus, or tissue biopsy.

     • Detects smear-negative cases and allows drug susceptibility testing.

     • Solid media: Lowenstein–Jensen, Ogawa, Middlebrook 7H10/7H11.

     • Liquid media: MGIT™ 960, Bactec™ 9000MB.

     • Higher sensitivity (10–10² CFU/ml) but slow growth (up to 8 weeks).

     • Requires skilled personnel and proper biosafety infrastructure.

  3. Serological assay (TST/Mantoux) – identifies delayed-type hypersensitivity.

  4. IGRA – measures IFN-γ response to TB-specific antigens.

  5. NAAT (GeneXpert MTB/RIF) – rapid molecular detection of TB and resistance markers.

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Diagnostic Test Performance and Availability (Summary Table)

Test	Sensitivity (%)	Specificity (%)	Availability in Indonesia
NAAT-TB detection	85	97	Limited to some labs
NAAT (GeneXpert MTB/RIF)	>92	>99	Limited to tertiary hospitals
AFB microscopy	70	≥90	Widely available
Growth detection – Liquid	90	99	Referral hospitals / some labs
Growth detection – Solid	88	99	Referral hospitals / some labs
DNA probe/HPLC identification	99	100	Research labs only
First-line drug susceptibility (liquid)	≥97	≥97	Referral hospitals / some labs
Second-line drug susceptibility (liquid)	≥92	≥97	Referral hospitals / some labs
Second-line drug susceptibility (solid)	82–99	92–100	Referral hospitals / some labs

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Tuberculosis (TB) Diagnosis in Indonesia

• General Diagnostic Approach:

  • In Indonesia, TB diagnosis is established based on a combination of clinical assessment, laboratory findings, and supporting examinations.

  • The main laboratory tests include:

    • Direct microscopic examination of sputum smears for Acid-Fast Bacilli (AFB).

    • Culture tests, using either:

      • Solid media (Lowenstein–Jensen / LJ), or

      • Liquid media (Mycobacteria Growth Indicator Tube / MGIT™ system).

    • Rapid molecular tests, particularly the GeneXpert MTB/RIF assay, which detects M. tuberculosis and rifampicin resistance.

      • Note: This test is used for diagnosis, not for treatment monitoring.

  • Supporting examinations may include radiological imaging (chest X-ray) and histopathological analysis when extrapulmonary TB is suspected.

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National Diagnostic Workflow (Ministry of Health, Indonesia)

• The Indonesian Ministry of Health provides a standardized diagnostic algorithm for TB, emphasizing stepwise evaluation and case confirmation:

  • Step 1 – Clinical Evaluation:

    • Adult patients presenting with a productive cough lasting ≥2 weeks undergo a detailed clinical examination.

  • Step 2 – Sputum Microscopy:

    • Patients are instructed to submit three sputum samples for microscopic AFB examination.

    • If any one sample is positive, the diagnosis of TB is confirmed, and treatment is initiated.

  • Step 3 – Further Testing (if microscopy negative):

    • If microscopy results are negative but clinical suspicion remains high, the patient should be referred for:

      • Chest X-ray, and/or

      • Additional tests, such as a rapid molecular assay (GeneXpert MTB/RIF) or culture.

    • A suggestive chest X-ray may justify starting TB treatment even when smear results are negative.

  • Step 4 – Limited Access Situations:

    • In healthcare settings where referral or advanced diagnostics are not possible:

      • The patient may receive a trial of broad-spectrum antibiotics.

      • Re-evaluation is conducted after treatment:

        • If symptoms improve, TB is unlikely (non-TB diagnosis).

        • If symptoms persist, repeat clinical and sputum microscopic assessments are warranted.

  • Step 5 – Drug Susceptibility Testing (DST):

    • If M. tuberculosis is successfully cultured, DST should be performed to guide effective treatment and detect potential drug resistance.

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Challenges and National Response

• Despite the presence of structured diagnostic systems, many TB cases remain unreported in Indonesia.

• To address this, several strategic measures have been implemented:

  • Mandatory TB case notification for all health facilities, including public and private hospitals, to ensure comprehensive case reporting.

  • Strengthened collaboration between healthcare sectors to identify and record patients under TB treatment.

  • Integration of GeneXpert testing into the national TB prevalence survey, enhancing case detection accuracy.

    • Previously, the survey relied on:

      • Screening for cough ≥2 weeks, and

      • Diagnosis based solely on microscopy.

    • Since 2013–2014, improved methods have been adopted:

      • Screening includes both symptom assessment and X-ray findings.

      • Diagnosis incorporates smear microscopy, culture, and GeneXpert confirmation for smear-positive samples.

• As a result of active case finding and mandatory notification, TB detection rates have significantly improved nationwide since 2016.

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Source: Susilawati, T.N. and Larasati, R., 2019. A recent update of the diagnostic methods for tuberculosis and their applicability in Indonesia: a narrative review. Medical Journal of Indonesia, 28(3), pp.284-91.