Diagnostic Techniques In Tuberculosis

Diagnostic Techniques In Tuberculosis


MANTOUX TUBERCULIN SKIN TEST(TST)

  • 0.1 ml of tuberculin purified protein derivative (-PPD) into inner surface of forearm intradermally. 
  • Most appropriate test to assess the prevalence of tuberculosis infection in a community
  • Read between 48-72 hrs.
  • Positive result is supportive evidence in the diagnosis of TB in areas of low prevalence (or no vaccination);

Negative result may occur in approximately one third of patients  could result from the following:

  • Anergy secondary to immunosuppression or malnourishment;
  • Recent infection;
  • Circulating mononuclear cells suppressing the specifically sensitized circulating T-lymphocytes in the peripheral blood; or
  • Sequestration of purified protein derivative specific reactive T-lymphocyte.

 False positive in Atypical mycobacterial infection and previous BCG vaccination.

  • False-negative tuberculin test is seen in
  • Miliary tuberculosis
  • Convalescence from some viral infections like Measles 
  • Lymphoreticular malignancy
  • Sarcoidosis 
  • Severe malnutrition 
  • Immunosuppressive therapy
  • Defective cell mediated immunity
CT SCAN:
  • Characteristic finding in CT in a TB case is Exudate seen in basal cistern
  • The Basal Cisterns are obscured by presence of inflammatory tissue exudate.
  • Hydrocephalus is communicating & is usually persistent i.e. it does not improve even if ATT is given.
  • Calcification is commonly seen in the meninges at the base of the brain.
  • Ventriculitis may be seen, but it is not common.

RADIOGRAPHY:

Findings:

  • Shadows in one or both upper zones
  • Cavities in one or both upper zones
  • Miliary pattern. 
  • Persistent shadows after pneumonia treatment. 
  • Pleural effusion. 
  • Intrathoracicadenopathy. 
  • Pericardial effusion
  • A combination of any of the above, especially in HIV-infected patients

SPUTUM TEST:

  • Sputum smears are usually positive in patients with laryngeal TB, endobronchial TB and cavitary pulmonary TBThe tiny droplets dry rapidly; the smallest 
  • There may be as many as 3000 infectious nuclei per cough.
  • Confirmatory test for tuberculosis 
  • As per RNTCP guidelines first do in TB suspect case
  • Rapid test used by WHO for tuberculosis i s Sputum AFB
  • AFB /ZN Technique 5min heated stain till cooling. 
  • Wash with clean water.5min for acid alcohol 1-2min for methylene blue.
  • Processing of sputum – use of NaOH 40g/L then centrifugation(18-23% more sensitive).

NUMBER OF BACILLI

RESULT
NO AFB(per 100 oil immersion field) 0
1-9 AFB(per 100 oil immersion field) Scanty
10-99 AFB(per 100 oil immersion field) +
1-10 AFB (per oil immersion field) ++
>10 AFB(per oil immersion field)  +++

MYCOBACTERIAL CULTURE:

  • Patient previously on anti-TB treatment (Relapse, Defaulter)
  • Still smear-positive after intensive phase of treatment or after finishing treatment
  • Symptomatic and at high-risk of MDR-TB
  • To test fluids potentially infected with M. tuberculosis
  • Investigation of patients who develop active PTB during or after IPT.
  • TB in health worker
  • LJ egg medium – protein enriched media with optimum temp of 35-37c.
  • LJ is an egg based media with addition of salts, 5% glyecerol and Malachite green(Inhibits growth of other bacteria).
  • Positive result (growth of mycobacteria) is usually apparent after three weeks.
  •  If there is no growth by 8 weeks, the result is negative. 
  • Approximately 4 weeks from receipt of specimen to culture results
  • An additional 3-4 weeks for susceptibility results
  • Therefore minimum of 7-8 weeks for DST results
  • Raised, dry cream (aka buff,rough and tough) coloured colonies.
  • Visible colonies are usually produced –weeks after incubation but culture should be incubated for upto 6weeks before being discarded.
  • Nitrate reduction and niacin production are definitive for M.tb

TB DRUG SUSCEPTIBILITY TESTING (DST)

  • BACTEC 460 TB System (radiometric)
  • BACTEC 12B vial, utilize 14C labeled substrate (Palmitic acid).
  • On inoculation, mycobacteria, grow and release 14CO2 in 2-3 weeks
  • The BACTEC instrument measures quantitatively the radioactivity on a scale ranging from 0-999, as GI (Growth Indicator).
  • When ATT is introduced in the medium, reduced production of 14CO2 & decrease in GI.
 NEW APPROACHES IN DIAGNOSIS:
  • Anitgen detection
  • LAM ELISA urinary antigen
  • Sputum antigen
  • Microscopy

Culture :

  • MODS
  • Phage test
  • Calorimetric media
  • Thin Agar media
  • NAATs
  • Volatile Organic Compound detection
  • PCR
  • Molecular Beacon Assay
  • GeneXpert MTB/RIF(used to detect INH and RIF resistance mutations)(fastest method 1-2 hr)
  • RT-PCR
  • Line Probe Assays
  • Cytokine Assays

D/D TESTS:

  • Mycobacterium tuberculosis is differentiated from other atypical mycobacteria by niacin test & Aryl sulphatase test

Exam Question

 
  • Tuberculin positive means Patient is infected with mycobacterium
  • 3000 infectious nuclei per cough are likely to be seen in sputum examination
  • Mycobacterium tuberculosis is differentiated from other atypical mycobacteria by niacin test & Aryl sulphatase test
  • The most appropriate test to assess the prevalence of tuberculosis infection in a community is tuberculin /Montoux test
  • False-negative tuberculin test is seen in  Miliary tuberculosis, Convalescence from some viral infections like Measles , Lymphoreticular malignancy, Sarcoidosis,  Severe malnutrition,  Immunosuppressive therapy, Defective cell mediated immunity
  • Characteristic finding in CT in a TB case is Exudate seen in basal cistern
  • Confirmatory test for tuberculosis is AFB
  • Fastest method for diagnosis of TB Gene expert
  • Sputum smears are usually positive in patients with laryngeal TB, endobronchial TB and cavitary pulmonary TB
  • Radiomimetric BACTEC detect growth of M tuberculosis in 2-3 week
  • Malachite green in LJ media Inhibits growth of other bacteria
  • As per RNTCP guidelines first do in TB suspect case is sputum microscopy
  • Rapid test used by WHO for tuberculosis i s Sputum AFB
  • Gene expert used for getting diagnosis of TB in 1-2 hrs
  • L j medium is used for growth of M. tuberculosis
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