radiographic examination of the chest is usually the first diagnostic study undertaken, after the history and physical examination .
In primary tuberculosis, as a recent infection, is seen as a middle or lower lung zone infiltrate , often associated with ipsilateral hilar adenopathy
. Atelectasis may result from compression of airways by enlarged lymph nodes. If the primary process persists beyond the time when specific cell-mediated immunity develops, cavitation may occur (so-called progressive primary tuberculosis).
Tuberculosis that develops many years after the original infection (endogenous reactivation) usually involves the upper lobes of one or both lungs.
Cavitation is common in this form of tuberculosis.
The most frequent sites are the apical and posterior segments of the right upper lobe and the apical-posterior segment of the left upper lobe.
Healing of the tuberculous lesions usually results in development of a fibrotic scar with shrinkage of the lung parenchyma and, often, calcification.
Involvement of the anterior segments alone is unusual. In the immunocompetent adult with tuberculosis, intrathoracic adenopathy is uncommon.
When the disease progresses, infected material may be spread via the airways (i.e., "bronchogenic" spread) into the lower portions of the involved lung or to the other lung. Erosion of a parenchymal focus of tuberculosis into a blood or lymph vessel may result in dissemination of the organism and a miliary pattern on the chest film .
Active tuberculosis (TB) is diagnosed by detecting Mycobacterium tuberculosis complex bacilli in specimens from the respiratory tract (pulmonary TB) or in specimens from other bodily sites (extrapulmonary TB).
acid fast bacilli (AFB) smear microscopy and culture on Löwenstein-Jensen medium are still the“ gold standards ” for the diagnosis of active TB.
AFB smear microscopy and culture can also be used to monitor the effectiveness of treatment and can help to determine when a patient is less likely to be infectious.
Adenosine deaminase, also known as ADA, is an enzyme involved in the metabolism of purines. Its presence is needed for the breakdown of adenosine from food and for the turnover of nucleic acids in tissues.
Many articles and reviews have reported the utility of ADA determination in body fluids(spinal, pleural, ascitic,pericardial)
ADA determination is simple and cheap and also has a high positive predictive value, especially in high endemic countries.
The specificity is very high in fluids with a lymphocyte-to-neutrophil ratio higher than 0.75
The suggested laboratory cut-off of ADA activity is 40 U/L for pleural, peritoneal and pericardial fluids and 10 U/L for cerebrospinal fluid. However different laboratories have established different cut-offs varying between 33 and 50 U/L.
Basically, it uses the specific humoral and cellular immune responses of the host to infer the presence of infection or disease
Humoral immune responses detection:-
Early studies utilizing crude antigen preparations of M. tuberculosis showed seroreactive antibodies in TB patients. However, cross-reactions occurred in healthy individuals, elicited by commensal bacteria, environmental mycobacteria and BCG
Tuberculin skin test ▪ TST has been used to identify patients actively infected with TB, ▪ to measure the prevalence of infection in a community, and to select susceptible or high-risk patients for BCG vaccination . How to do TST : ▪ TST works by intradermally injecting 0.1 mL of 5 TU PPD on the forearm. (Mantoux's method) On examination, after 48-72 hours, a positive reaction is indicated by erythema and induration of > 10 mm in size. Erythema (redness) alone is not taken as a positive reaction . ▪ To detect persons at high risk for LTBI or at high risk to progress to disease
☻ A positive TST result is a manifestation of type IV delayed hypersensitivity. □ False-positive test ﬁndings can occur in ; ● cross-reactions caused by non-tuberculous (atypical) mycobacterial infection ● bacille Calmette-Gu´ erin (BCG) vaccination.
Causes of False-Negative Tuberculin Skin Test 1: Factors Releated to the Person Being Tested ● Concurrent infections ● Human immunodeficiency virus ● Other viral infections (measles, mumps, chickenpox) ● Bacterial diseases (typhoid fever, brucellosis, typhus, leprosy, pertussis, overwheiming tuberculosis) ● virus vaccination (measles, mumps, polio) ● Diseases affecting lymphoid organs (Hodgkin's disease, lymphoma, chronic lymphocytic leukemia, sarcoidosis) ● Age (newborns, elderly) ● Metabolic conditions (advanced renal failure, malnutrition) ● Recent surgery ● Certain medications (e.g.,TNF- α antagonists, corticosteroids and other immunosuppressive drugs), will suppress the IV response and T-lymphocyte function
2 : Factors Related to the Test ● Loss of antigen potency (exposure to heat and/or light) ● Incorrect administration (too little antigen, deep injection) ● Incorrect reading (inexperienced reader, reader bias, recording error
-Compared with TST these tests have higher specificity.
-Correlate better with exposure to tuberculosis.
-Have less cross-reactivity with the BCG vaccine & environmental mycobacteria.
NB: Yet there is no evidence for the use of these tests in young children at present.
Enzyme-linked immunospot for interferon-gamma The ELISPOT assay for diagnosis of M. tuberculosis infection is based on the rapid detection of T cells specific for M. tuberculosis antigens. IFN-γ released in vivo from these cells can be detected by the extremely sensitive ELISPOT In a preliminary trial, this test was positive in 96 % of 47 TB patients and in 85 % of 26 persons presumed to have latent TB. The ELISPOT test was negative in 26 BCG-vaccinated control subjects, and this specificity implies a major advantage over TST ELISPOT offers a more accurate approach than TST for the identification of individuals who have latent TB infection . Each such T cell gives rise to a dark spot and the readout is the number of spots. The T cells enumerated by the ELISPOT assay are effector cells that have recently encountered antigen in vivo and can rapidly release IFN-γ when re- exposed to the antigen
Quantiferon-TB test The IFN-γ secreted by T-cells into the plasma is measured by ELISA to indicate the likelihood of TB infection . Different studies demonstrated that the QuantiFERON-TB test was comparable to TST in its ability to detect latent TB infection. also showed that the QuantiFERON-TB test was less affected by BCG vaccination QuantiFERON-TB was approved by the Food and Drug Administration (FDA) of the United States (US) in 2001. In 2003, the US Centers for Disease Control and Prevention released guidelines for using the QuantiFERON®-TB Test in the diag- nosis of latent M. tuberculosis infection ,
QuantiFERON-TB Gold , This in vitro test detects the release of interferon-gamma (IFN-γ) from lymphocytes of sensitized persons when their blood is incubated with peptide mixtures simulating two M. tuberculosis proteins called ESAT-6 and CFP-10 . the TB-specific antigens ESAT-6 and CFP- 10 that are only present in M. tuberculosis and are absent from all strains of M.bovis (BCG) and most environmental mycobacteria ,
LIMITATIONS IFN γ assays are expensive tests and their higher cost appears to limit their wider applicability, especially in resource-limited settings and developing countries,. The ELISPOT test is not yet suitable for widespread use, because it is costly and requires isolation of mononuclear cells, a procedure that is not performed in clinical laboratories .
Members of M.tuberculosis complex do not grow in the presence of p-nitro- ∞ -acetylamine
If 5 ug of NAP is added to actively growing culture in 12B medium vial the growth of M. tuberculosis complex is inhibited while Mycobacteria other than tuberculosis (NTM) do not demonstrate significant inhibition.