Question 12

a)    With respect to the diagnosis of pulmonary tuberculosis, describe what investigations are available in ICU patients and their advantages and limitations.    (50% marks)

b)    What infection control precautions should be taken when admitting a patient with active pulmonary tuberculosis to the ICU?    (50% marks)

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College answer

Not available.


Investigation Advantages Limitations
  • Gold standard
  • Also provides drug susceptibility information
  • Only 100 or so organisms are required for a culture
  • Takes an extremely long time
AFB identification (ZN stain)
  • Next best thing to gold standard
  • Relatively fast compared to culture
  • Induced sputum or bronchoscopy speciments are required for maximum sensitivity 
  • Requires a high pathogen load: there must be 5,000 to 10,000 bacilli per ml 
Nucleic acid amplification
  • Very rapid (couple of hours)
  • Can be performed on many different specimen types
  • Requires as few as 10 bacilli
  • High specificity: if its positive, its positive.
  • Effective at discriminating M.tuberculosis from other mycobacteria, which AFB may not.
  • Sensitivity is poor: may return a negative result in ~50% of patients with culture-positive tuberculosis
  • Does not replace AFB smear and culture
  • Remains positive for months after TB is successfully treated
Skin tuberculin test
  • Widely available, including resource-poor environments
  • Does not require a dedicated laboratory
  • High positive predictive value in areas of high TB community prevalence
  • Slow
  • Poor sensitivity and specificity in areas of low TB prevalence
  • Unclear what to do with intermediate results
Line probe assay
  • A rapid test for drug sensitivity which can detect gene markers associated with drug resistance
  • Cannot replace conventional culture and sensitivity testing
  • Only appropriate for patients already tested positive
Interferon-γ release assays
  • Quick and convenient screening tool for exposure to TB
  • Cannot discriminate between latent and active TB
  • Does not give drug sensitivity data
Xray or CT imaging
  • Presence of cavitating lesions strongly raises suspicion of TB
  • Guide for bronchoscopy or imaging-guided aspiration of more diagnostic material
  • Convenient mechanism of monitoring disease activity when the diagnosis is already established
  • Not diagnostic of TB per se, only raises suspicion
  • Requires patient transport and radiation exposure
  • Requires radiology staff to be exposed to TB patient
  • Pathological diagnosis of a caseating granuloma in an anatomical specimen is diagnosit
  • Tissue can be cultured and tested with NAA
  • Highly invasive
  • Requires a good quality specimen
  • Not widely avalable

Infection control precautions listed here have been pulled directly from the CDC document, where most people get their infection control guidelines from:

  • Airborne transmission precautions:
    • Should be commenced as soon as a patient is suspected of having TB
    • N95 masks for staff attending directly to patient care, and any visitors
    • Surgical mask for the TB-positive patient when in transport or in waiting areas
    • Single room
    • Airflow of six or more air changes per hour (ideally, 12)
    • HEPA filtration of any recirculated air
    • Negative pressure system is desirable
    • Private bathroom is desirable if the pt. is conscious
    • Minimise aerosol-generating procedures;
    • Minimise staff involved in any necessary aerosol-generating procedures
  • Clearing of the infectious status:
    • Patients can be considered noninfectious when they meet all of the following three criteria:
      • They have three consecutive negative AFB sputum smears collected in 8- to 24-hour intervals (at least one being an early morning specimen);
      • Their symptoms have improved clinically (for example, they are coughing less and they no longer have a fever);
      • They are compliant with an adequate treatment regimen for 2 weeks or longer.


Chapter 73  (pp. 743) Tropical  diseases  by Ramachandran  Sivakumar  and  Michael  E  Pelly

Erbes, Reinhard, et al. "Characteristics and outcome of patients with active pulmonary tuberculosis requiring intensive care." European Respiratory Journal27.6 (2006): 1223-1228.

Hepple, P., N. Ford, and R. McNerney. "Microscopy compared to culture for the diagnosis of tuberculosis in induced sputum samples: a systematic review [Review article]." The International Journal of Tuberculosis and Lung Disease16.5 (2012): 579-588.

Hagan, Guy, and Nazim Nathani. "Clinical review: Tuberculosis on the intensive care unit." Critical Care 17.5 (2013): 240.

Ryu, Yon Ju. "Diagnosis of pulmonary tuberculosis: recent advances and diagnostic algorithms." Tuberculosis and respiratory diseases 78.2 (2015): 64-71.