Diagnosis of Tuberculosis
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Questions and Answers

What is a critical advantage of using phenotypic drug resistance determination in TB diagnosis?

  • It can identify a wide range of drug resistances. (correct)
  • It requires no specialized laboratory skills.
  • It is less expensive than genotypic testing.
  • It provides results faster than molecular methods.

    • Which of the following methods is primarily used for genotypic drug resistance determination in TB?

  • Microscopy
  • Culture-based methods
  • Xpert MTB/RIF assay (correct)
  • Urine lipoarabinomannan assay

    • What is a major limitation of current diagnostic tests for latent TB infection?

  • They can only identify active TB cases.
  • They cannot differentiate between latent and active TB. (correct)
  • They are only effective for pulmonary TB.
  • They require a long laboratory processing time.

    • Which platform is recommended for diagnosing latent TB according to national guidelines?

    <p>Interferon gamma release assays (IGRAs)</p> Signup and view all the answers

    What is a significant disadvantage of direct microscopy in TB diagnosis?

    <p>It has low sensitivity compared to culture methods.</p> Signup and view all the answers

    What is one of the primary challenges of transporting TB specimens?

    <p>Minimizing overgrowth of fast-growing contaminants.</p> Signup and view all the answers

    How does the Hain Line Probe assay compare to Xpert MTB/RIF assay in sensitivity?

    <p>Xpert MTB/RIF assay is more sensitive.</p> Signup and view all the answers

    What specimen type is essential for the diagnosis of pulmonary TB?

    <p>Sputum samples</p> Signup and view all the answers

    What is a primary benefit of genotypic methods in drug susceptibility testing for TB?

    <p>They provide faster results and can detect specific mutations.</p> Signup and view all the answers

    Which statement best describes a limitation of using urine lipoarabinomannan assay?

    <p>It is highly specific but not very sensitive.</p> Signup and view all the answers

    Which of the following methods allows for precise identification of mycobacterial species?

    <p>Culture methods</p> Signup and view all the answers

    What is a primary advantage of using the Gene Xpert MTB/RIF assay for drug susceptibility testing?

    <p>It detects a larger repertoire of drugs.</p> Signup and view all the answers

    What is a significant limitation of direct microscopy when diagnosing active TB disease?

    <p>It has a low sensitivity of 25% - 65%.</p> Signup and view all the answers

    Which of the following limitations is associated with genotypic methods for drug susceptibility testing?

    <p>They may miss resistance-inducing mutations.</p> Signup and view all the answers

    What does phenotypic drug susceptibility testing compare?

    <p>Bacterial culture in the presence and absence of drugs.</p> Signup and view all the answers

    Which specimen is NOT acceptable for blood culture when diagnosing TB?

    <p>A pooled 24-hour specimen.</p> Signup and view all the answers

    What is a significant drawback of the Hain Lifescience Line probe assay (LPA)?

    <p>It requires complex laboratory infrastructure.</p> Signup and view all the answers

    What aspect does genotypic drug susceptibility testing primarily detect?

    <p>Specific mutations conferring drug resistance.</p> Signup and view all the answers

    Why is the turn-around time (TAT) for the Hain LPA generally longer than for Gene Xpert assays?

    <p>It involves a more intricate process and expertise.</p> Signup and view all the answers

    Which limitation is common between direct microscopy and culture methods?

    <p>Inability to differentiate species.</p> Signup and view all the answers

    Which of the following statements is true regarding the use of PCR-based platforms for monitoring treatment response?

    <p>They may falsely suggest resistance due to residual DNA.</p> Signup and view all the answers

    What is the minimum cerebrospinal fluid volume required for TB testing?

    <p>5 ml</p> Signup and view all the answers

    In the context of tuberculosis management, what is a limiting factor of genotypic testing methods?

    <p>They require advanced laboratory facilities.</p> Signup and view all the answers

    What distinguishes the Gene Xpert MTB/RIF Ultra assay from the original Gene Xpert MTB/RIF assay?

    <p>It enables detection of a wider range of drug resistance.</p> Signup and view all the answers

    What factor significantly influences the sensitivity of direct microscopy for TB diagnosis?

    <p>Specimen quality and site of collection.</p> Signup and view all the answers

    Which of the following is essential to maintain during the collection of tissue and fluids for TB diagnosis?

    <p>Collection in a sterile container without preservatives.</p> Signup and view all the answers

    For which types of specimens is the Gene Xpert MTB/RIF assay typically performed?

    <p>Pulmonary and selected extra-pulmonary specimens.</p> Signup and view all the answers

    What does the mycobacteria growth indicator tube (MGIT) provide in the context of TB diagnosis?

    <p>Liquid culture media for mycobacteria.</p> Signup and view all the answers

    Which of the following is a specific shortcoming of drug susceptibility testing via genotypic methods?

    <p>They may not be able to monitor the effectiveness of ongoing treatment.</p> Signup and view all the answers

    What is a key advantage of implementing drug susceptibility testing in the management of tuberculosis?

    <p>It ensures appropriate treatment regimens are selected.</p> Signup and view all the answers

    What is a significant limitation of the Tuberculin Skin Test (TST)?

    <p>It may show false negatives in immunocompromised individuals.</p> Signup and view all the answers

    Which characteristic makes IGRA more specific than TST?

    <p>It utilizes specific injected antigens unique to M.tuberculosis.</p> Signup and view all the answers

    Under which condition is the Lateral Flow Urine LAM test particularly beneficial?

    <p>For point-of-care testing in active TB disease.</p> Signup and view all the answers

    What is the primary principle behind diagnosing latent TB infection?

    <p>Observation of skin response to injected antigens.</p> Signup and view all the answers

    What does a positive IGRA result indicate?

    <p>Prior exposure or infection with M.tuberculosis.</p> Signup and view all the answers

    Which of the following statements about the limitations of TB diagnostic tests is accurate?

    <p>TST results can be subjective and influenced by other mycobacterial infections.</p> Signup and view all the answers

    In the context of drug susceptibility testing methods, what is essential to determine?

    <p>The specific strains of M.tuberculosis present.</p> Signup and view all the answers

    Why is the urine LAM test particularly useful in patients co-infected with HIV?

    <p>It shows improved sensitivity with lower CD4 counts.</p> Signup and view all the answers

    What approach is typically used to confirm active TB disease after initial screening?

    <p>Conducting microbiological tests.</p> Signup and view all the answers

    Which diagnostic method can provide immediate results at the point of care for active TB?

    <p>Lateral flow urine lipoarabinomannan assay.</p> Signup and view all the answers

    Study Notes

    Diagnosis of TB

    • This presentation is about diagnosing Tuberculosis (TB)
    • The presenter is Dr Marianne Black
    • The presenter works at the Mycobacteriology Referral Laboratory in Braamfontein, Johannesburg
    • She works at the Department of Clinical Microbiology and Infectious Diseases at the University of the Witwatersrand
    • The date of presentation is July 2019

    Lecture Objectives

    • List specimen types suitable for TB investigations
    • List advantages and limitations of direct microscopy for TB diagnosis
    • List advantages and limitations of culture as a diagnostic method for TB
    • List advantages and limitations of phenotypic drug resistance determination in TB
    • List advantages and limitations of genotypic drug resistance determination in TB
    • Compare Xpert MTB/RIF and Hain Line Probe assays
    • Grade TB diagnostic platforms based on increasing sensitivity
    • Explain the principle of latent TB diagnosis and list available platforms
    • List limitations of existing tests for latent TB diagnosis
    • Describe the principle of urine lipoarabinomannan assay and WHO recommendations

    Specimen Collection and Transport

    • Specimen Collection:
      • Safety for healthcare workers (HCWs)
      • Clear labeling
      • Good quality, representative specimens (early morning preferred, but not always feasible)
    • Specimen Transport:
      • Safe packaging to minimize leakage/contamination
      • Delivery to the lab within 4 hours
      • Maintain cold chain, out of direct sunlight
      • Important to avoid overgrowth of other bacteria
      • High temperatures can kill bacilli (important for culture of M. tuberculosis)
      • Transportation should be on ice if necessary
    • Specimen Types (for Culture):
      • Pulmonary TB: Sputum, induced sputum, gastric aspiration, bronchial washings, bronchoalveolar lavage (BAL)
      • Disseminated TB/Isolated Organ TB: Blood, cerebrospinal fluid (CSF), urine, lymph node aspirate, tissue, other body fluids, bone marrow, skin samples

    Specimen Collection and Transport (Specific Details)

    • Sputum: Sterile, wide-mouthed containers, induced sputum (nebulized hypertonic saline) - strict airborne precautions
    • Gastric Aspirates: Early morning aspirate after fasting 4-6 hours
    • Urine: First morning, clean catch, midstream urine (minimum 40 ml), 24-hour pooled specimens not acceptable
    • Blood Culture: Aseptic collection, inoculated 5 ml at the bed site into broth media, BACTEC MyocoF Lytic
    • Cerebrospinal Fluid (CSF): Minimum of 5 ml
    • Tissue and Fluids: (e.g., lymph node/pleural fluid aspirate/biopsy), sterile container with no fixative or preservative

    Active TB Disease Diagnosis

    • Direct Microscopy: Auramine smear
    • Culture: Specialized liquid culture media (mycobacteria growth indicator tube - MGIT)
    • Drug susceptibility testing:
      • Phenotypic methods
      • Genotypic methods

    Direct Microscopy (Advantages and Limitations)

    • Advantages: Inexpensive, rapid results, quantitative measure of bacterial load (smear grade correlates with infectiousness - negative, scanty, 1+, 2+, 3+), treatment monitoring
    • Limitations: Low sensitivity (25%-65%), visual detection limit is 10,000 bacilli/ml, influenced by specimen quality (sputum vs. saliva, site, HIV status). Cannot differentiate mycobacterial species (MTB complex vs. NTM), Cannot differentiate between viable/nonviable bacilli, Cannot determine drug susceptibility

    Culture (Advantages and Limitations)

    • Advantages: Gold standard, high sensitivity (10-100 bacilli/ml), precise species identification, allows for (culture-based) phenotypic drug susceptibility testing, useful to monitor treatment response
    • Limitations: Mycobacteria divide once/day - lengthy time to result, prone to contamination (bacteria/fungi may outgrow mycobacteria), requires biosafety level 3 laboratory infrastructure

    Drug Susceptibility Testing (DST)

    • Phenotypic (culture-based) methods: Comparison of growth in the presence and absence of the drug, gold standard
    • Genotypic methods: Detect mutations in the DNA sequence that confer resistance
    • Gene Xpert MTB/RIF assay (GXP) / Xpert® MTB/RIF Ultra, Hain Lifescience® Line probe assays (LPA)

    Gene Xpert MTB/RIF Assay

    • Identify MTB complex and provides rifampicin susceptibility result
    • Performed on pulmonary and selected extra-pulmonary specimens (e.g., purulent fluid, tissue, CSF)
    • Turn-around time of 24 hours

    Xpert MTB/RIF Ultra Assay

    • Concentrates bacilli and removes inhibitors
    • Sample is filtered and washed
    • Ultrasonic lysis of filter to release DNA
    • DNA mixed with dry PCR reagents
    • Semi-nested real-time amplification and detection in integrated reaction tube
    • Time-to-result: 1 hour 45 minutes
    • Printable test result
    • Minimal expertise needed
    • Turnaround time (TAT) of 24 hours (laboratory-based)

    Drug Susceptibility Testing (Genotypic methods - LPA)

    • Hain Line probe assay (LPA) MTBDRplus and MTBDRsl
    • Detects MTB complex
    • 1st line LPA: rifampicin and isoniazid
    • 2nd line LPA: fluoroquinolones and second-line injectable agents
    • Performed on clinical specimens and culture
    • More complex process, expertise needed, laboratory infrastructure
    • TAT: 2-3 days

    Latent TB Infection Diagnosis

    • Principle: Detect immune response (cell-mediated immunity) to previous infection with M. tuberculosis
    • Available platforms:
      • Skin: Induration in response to inflammation (tuberculin skin test [TST] or purified protein derivative [PPD], Mantoux test)
      • Blood: Production of interferon-gamma (IFN-γ) by white blood cells (interferon gamma release assays [IGRAs])

    Latent TB Infection Diagnosis - Limitations

    • Cannot differentiate between latent TB infection and active TB disease
    • May be positive in both disease and infection
    • Requires further microbiological testing
    • False negatives may occur in immunocompromised or advanced TB disease
    • Limited use in highly endemic countries
    • TST: Time-consuming, subjective, less specific than IGRA
    • IGRA: Expensive, but more specific than TST (injected antigens specific to M. tuberculosis)

    Lateral Flow Urine Lipoarabinomannan Assay (Urine LAM Test)

    • Based on detection of mycobacterial lipoarabinomannan (LAM) antigen in urine
    • Released from metabolically active or degenerating mycobacterial cells
    • Point-of-care test for TB
    • LAM only present in people with active TB disease
    • Urine is easy to collect and store, and lacks infection control risks associated with sputum collection
    • Improved sensitivity in HIV-TB co-infection (lower CD4 counts, especially below 100 cells/µL)
    • Limitations: Not good for general screening, negative result does not exclude TB (other diagnostic tests needed), does not differentiate species of mycobacteria

    WHO Recommendations for Urine LAM

    • Inpatient Settings: WHO strongly recommends using LF-LAM to assist in the diagnosis of active TB in HIV-positive adults, adolescents, and children with signs and symptoms of TB, advanced HIV disease or seriously ill, or with a CD4 count of less than 200 cells/mm3
    • Outpatient Settings: WHO suggests using LF-LAM to assist in the diagnosis of active TB in HIV-positive adults, adolescents, and children with signs, symptoms of TB or seriously ill, or with a CD4 count of less than 100 cells/mm3
    • WHO recommends against using LF-LAM in outpatient settings without assessing TB symptoms or without TB symptoms and a CD4 count above 200 cells/mm3

    Limit of Detection

    • Microscopy: 5,000-10,000 bacilli/mL
    • MTBDRplus: 160 bacilli/mL
    • Xpert MTB/RIF: 130 bacilli/mL
    • Culture: 10-100 bacilli/mL
    • GXP Ultra: 10-100 bacilli/mL

    TB Diagnostic Algorithm

    • Refer to presentation for the detailed algorithm

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    Description

    This quiz covers the fundamentals of diagnosing Tuberculosis (TB), presented by Dr. Marianne Black from the University of the Witwatersrand. Participants will explore various diagnostic methods, specimen types, advantages, and limitations involved in TB investigations. Test your knowledge on contemporary TB diagnostic platforms and drug resistance determination techniques.

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