**4. Patho-physiological assessment in TB diagnosis**

Pathological and or physiological changes in the body can be used to diagnose mycobacteria infections or TB. Two common in vivo methods, TST, and chest radiography are presented in this chapter.

### **4.1. Tuberculin skin test (TST)**

The TST is the internationally recognized standard method to identify infection with *M*. *tuberculosis* and *M. bovis*. Intradermal tuberculin tests based on eliciting a delayed-type hypersensitivity response, which is mediated by a population of sensitized T-cells and takes some weeks to develop after infection [52]. The TST use the purified protein derivative (PPD) tuberculins that have been derived from *M. bovis* and *M*. *tuberculosis* for detection of latent TB in cattle and in human, respectively.

Techniques that are used for tuberculin skin testing in human include; Multiple puncture tests (tine test, heaf test, and MONO-VACC test) and intracutaneous injections either given by the jet injector or by the Mantoux test [53]. Multiple puncture tests introduce tuberculin into the skin through tuberculin-coated prongs while the Mantoux skin test involves the intracutaneous injection of tuberculin into the volar surface of the forearm [54]. The Mantoux test is recommended for clinical assessment or screening since it is more sensitive and specific than the other methods. The reported sensitivity and specificity of TST in human ranges from 59 to 100% and 70 to 100%, respectively [53].

There are several factors that are associated with false negative results of intradermal skin test in humans. These include recent or advanced TB, severe malnutrition, immuno deficiency due to the HIV or immunosuppressive chemotherapy, co-infections with viral, bacterial and fungal diseases affecting lymphoid organs (lymphoma, chronic lymphoid leukemia sarcoidosis), metabolic derangement (chronic renal failure), stress (surgery, burns, and mental illness), and live vaccination (measles, mumps, and polio) [53, 54]. False positive reactions are more commonly attributed to co-infection or pre-exposure to other related NTM in cattle and human [55] and BCG vaccination in human [56].

Despite of the several major limitations, TST has been, until recently, a useful procedure for detecting LTBI [57] and most widely used test for control of Bovine TB due to the low-cost, low logistical demands, well-documented use, and its ability to screen the entire cattle populations [52]. Due to the limitations of the sensitivity and specificity of the skin tests, newer ancillary in vitro diagnostic assays for latent tuberculosis has been developed, which measures the production of interferons in whole blood upon stimulation with PPD [52].

#### **4.2. Imaging in TB diagnosis**

and 73.9% accuracy [47]. Sample evaluation results are potentiated by use of multiple rats (e.g. group of four) and positive indication by at least one rat criteria has been reported to be

*Cricetomys gambianus* are resistant to TB infection [48]. The rats can detect mycobacteria in sputum in as low concentration as a few bacteria in less than 10 sec [49]. In contrast to a trained microscopist who can examine less than 50 samples a day, the rats can detect hundreds of samples per day before exhaustion [50]. These features make sputum sniffing by the giant African rats a potential rapid screening test. Such technique has been used in Tanzania and Mozambique as a second line screening after ZN staining sputum smear microscopy where 44% increase in TB case detection rate has been reported [48]. However, sputum sniffing by the African giant rats does not guarantee 100% accurate TB diagnosis. Although this disqualifies it as a stand-alone TB diagnostic or first line screening test for presumptive test it can be useful in increasing TB cases among smear-negatives especially in low-income TB

Moreover, TB diagnosis by sniffing rats works in active TB, it may not be handy in latent TB cases. Furthermore, the rats are living creature whose health status needs attention and any

Pathological and or physiological changes in the body can be used to diagnose mycobacteria infections or TB. Two common in vivo methods, TST, and chest radiography are presented in

The TST is the internationally recognized standard method to identify infection with *M*. *tuberculosis* and *M. bovis*. Intradermal tuberculin tests based on eliciting a delayed-type hypersensitivity response, which is mediated by a population of sensitized T-cells and takes some weeks to develop after infection [52]. The TST use the purified protein derivative (PPD) tuberculins that have been derived from *M. bovis* and *M*. *tuberculosis* for detection of latent TB in cattle and

Techniques that are used for tuberculin skin testing in human include; Multiple puncture tests (tine test, heaf test, and MONO-VACC test) and intracutaneous injections either given by the jet injector or by the Mantoux test [53]. Multiple puncture tests introduce tuberculin into the skin through tuberculin-coated prongs while the Mantoux skin test involves the intracutaneous injection of tuberculin into the volar surface of the forearm [54]. The Mantoux test is recommended for clinical assessment or screening since it is more sensitive and specific than the other methods. The reported sensitivity and specificity of TST in human ranges from 59 to

There are several factors that are associated with false negative results of intradermal skin test in humans. These include recent or advanced TB, severe malnutrition, immuno deficiency

more sensitive than multiple positive indications [48].

10 Basic Biology and Applications of Actinobacteria

endemic countries where diagnostics capabilities are limited [51].

**4. Patho-physiological assessment in TB diagnosis**

this chapter.

**4.1. Tuberculin skin test (TST)**

in human, respectively.

100% and 70 to 100%, respectively [53].

anatomical, physiological, or functional deformity may impair TB diagnosis.

Radiography uses electromagnetic radiations (x-rays) to visualize internal body organs. Chest radiography is used for imaging lungs, airways, ribs, heart, and diaphragm. The rays are allowed to pass through tissues and captured by a film. Variation of absorption of x-rays by different tissues enables contrasting visualization. Hard tissues, for example, bones absorb more rays and appear dense while soft tissues allow more passage of rays and appear light. Knowledge of anatomy is important for diagnostic interpretation.

According to WHO [58] chest radiography is indicated as a pulmonary TB diagnostic tool for triaging, screening and as a diagnostic aid when clinical pulmonary TB cannot be confirmed bacteriologically. Chest radiography can also be used in the evaluation of TB treatment response. It is, therefore, part of an algorithm within health care system. It can be used to distinguish between active and inactive pulmonary TB based on a temporal evaluation of radiographs whereby the latter is characterized by stable radiographic pictures for 6 months [59].

Most common chest radiographic findings of TB include lymphadenopathy, parenchymal opacities, obstructive atelectasis, pleural effusion, cavitations, and tuberculomas [59]. Chest radiography has high-sensitivity (97%, 95% CI 0.90–1.00) and low-specificity (67%, 95% CI 0.64–0.70) [60]. It can detect any abnormality in the chest including those related to other non-TB pathological conditions. Other imaging techniques such as computed tomography (CT) and Magnetic resonance imaging (MRI) are superior to chest radiography and can be used for reference. It is important to note that besides the low specificity, chest radiography is expensive in terms of equipment and skilled labor, especially in low-income endemic areas.
