diagnosis of tuberculosis Flashcards
what is the diagnosis of pulmonary TB based upon ?
clinical
radiological
tuberculin testing
how many sputum samples are required for a diagnosis of TB
3 consecutive morning sample
how is the detection of the intact bacilli done?
direct smear microscopy
culture and sensitivity
animal inoculation
what are the types of direct smear microscopy ?
classical Ziehl Neelsen method
fluorescent method
what are the causes of false positives smears ?
acid fast particles other than TB bacilli
- food particles
- precipitated stain
- contamination
what are the cause of false negatives smear ?
inadequate sputum collection
inadequate sputum storage
inadequate stained smear
what are the different medias used in culture of mycobacterium ?
egg based - LJ medium
agar based
liquid based
what is the problem with growing a culture ?
takes a long time 6-8 weeks do MGIT is now used
how is the detection of metabolic end product of bacilli done?
radiometric technology ( middle brook 7H12 )
measures radio labelled CO2
positivee culturee is above 10 GI
mycobactins
how are the fragments of bacilli detected ?
antigen tuberculo-stearicacid mycollic acid luciferin DNA or RNA
how would we detect mycobacterium using genetic probes (DNA, RNA) ?
NAAT-PCR
how can we detect drug resistance ?
we use PCR to identify drug resistance
INH resistance
rifampicin resistance
how can we detect the bacteria itself ?
either by detecting the intact bacillus itself
detecting fragments of the bacillus
or detecting metabolic end products
how do we detect immune response ?
serology
lymphocyte activity markers ( ADA and gamma INF)
tuberculin skin test
what is the cut off value of ADA in the CSF?
cutoff value is 8 U/l differentiates TB meningitis from other causes of lymphocytic meningitis
what is the cut off value of ADA in ascitic fluid ?
ADA above 36 U/L indicates possible TB
what is the cut off value of ADA in pleural fluid ?
- ADA level >70 U/l is highly suggestive of TB
- a level below 40 excludes a diagnosis
where is ADA released from ?
lymphocytes
if the ADA level in the pleura is between 40-70 what can we do to confirm a diagnosis of TB?
ADA ratio more than 2
how is interferon gamma used to diagnose a case of TB?
a sample of blood with previously sensitized lymphocytes is taken, antigens are introduced to the sample and the amount of gamma interferon made by the lymphocytes are measured
NOT SHARED BY BCG
What are the antigens thats are introduced to the blood sample in gamma interferon testing ?
ESAT6 and CFP10
CFP10
what is the immunological basis for tuberculin reaction ?
kochs phenomenon
what type of reaction happens with the tuberculin test ?
delayed hypersensitivity reaction
what is injected in the tuberculin test ?
PPP tuberculin, 5 units of tuberculin , 0.1 mm intradermally injected
how long does a result take to come out in a tuberculin skin test ?
48-72 hours later
how is the reaction of the tuberculin test measured ?
by the induration , no induration ? 0mm
what is the booster effect and what test is it positive in?
false positive with TST
how to diagnose a latent TB infection ?
TST or IGRA assay
IGRA assays
what is the medium used for the detection of metabollic end products ?
liquid middle brook 7H12
what is measured in the radiometric technique ?
carbon dioxide levels as growth index
what Growth index value indicates a positive culture ?
GI >10
what mutation is associated with reifampcin resistyance ?
rpoB gene
what are high level isoniazid resistance levels associated with ?
kat G gene
what are low level isoniazid resistance associated with ?
inhA gene
