Tuberculosis Testing (Blood Tests and Skin Tests), Research Paper Example

Introduction

Tuberculosis is rightly considered as one of the most serious infectious diseases in many countries. A highly contagious disease, tuberculosis (TB) presents a significant challenge to public health in many parts of the world. The difficulties with testing and diagnosing TB are well-documented: the absence of clinical symptoms in latent infection and the multitude of factors affecting the results of the TB test responses complicate the situation. Today, TB blood tests and the TB skin test (TST) are the two common procedures, used to identify and diagnose TB. The diagnostic and clinical significance of the TB blood and skin tests is difficult to underestimate. Nevertheless, both tests leave sufficient room for further improvement.

Tuberculosis Skin Test

The Mantoux tuberculin test is a standard instrument of diagnosing TB. The TB skin test (TST) is used to determine whether individuals have the Mycobacterium tuberculosis infection in their blood (Centers for Disease Prevention and Control, CDC). “The TB skin test is performed by injecting a small amount of fluid – called tuberculin – into the skin in the lower part of the arm” (CDC). The TST is performed with the help of a tuberculin syringe, and the exact amount of tuberculin administered during the skin test procedure is usually equal to 0.1 ml (CDC). It is an intradermal injection, which, when administered properly, must lead to a slight elevation of the skin, between 6 and 10 mm in diameter (CDC). The results of the Mantoux skin test are to be read within 48-72 hours, following an injection (CDC). Those who fail to attend a physician during 72 hours after the injection will have to repeat the skin test procedure (CDC). The interpretation of the TST results is based on the size of the skin induration and the extent, to which a person is subject to the risks of infection (CDC). For example, an induration of 5 to 10 millimeters is believed to be positive in HIV-infected individuals and persons with organ transplants (CDC). Simultaneously, an induration of 10 to 15 millimeters is considered positive in injection drug addicts and children in high-risk population groups (CDC). Unfortunately, false-positive and false-negative reactions in the TST are not uncommon. The principal causes of false-positive reactions include recent vaccinations (including BCG vaccinations), improper administration and interpretation, as well as the presence of nontuberculosis mycobacteria in blood (CDC). False-negative reactions are possible in individuals with recent or particularly old TB infections, viral illnesses, and after recent vaccinations (CDC). False-negative reactions can also be the result of incorrect interpretation (CDC). This is one of the reasons why TB blood tests are used as a routine procedure in TB diagnosis.

Blood Tests for TB

The current state of research confirms the positive predictive value of TB blood tests in detecting TB. Diel et al suggest that a whole blood IFN-? assay is a more reliable indicator of latent TB infection, compared with the conventional TST (1164). Unlike the TST, IFN-? release assays exemplify a valid instrument of predicting active TB in patients with silicosis; the latter is a common risk factor for active TB (Leung et al 839). Ultimately, the use of Interferon-Gamma release assays in elderly populations presents a cost-effective alternative to traditional Chest X-Ray screening (Kowada et al 235). Thus, it comes as no surprise that several types of the IGRA tests were approved by the Food and Drug Administration (CDC).

According to CDC, three types of IGRA approved for diagnosing TB include (a) QuantiFERON®-TB Gold In-Tube test, (b) Quanti-FERON®-TB Gold Test, and (c) T-SPOT®.TB (CDC). These blood tests measure the immune response to Mycobacterium tuberculosis in blood (CDC). That the IGRA tests cannot distinguish between latent and active forms of TB is one of their primary drawbacks and additional tests are required, to produce a relevant, correct diagnosis (CDC). Test results are interpreted, depending on the presence (positive) or absence (negative) of an immune response (CDC). Borderline results are possible, only when T-SPOT®.TB is used (CDC). These results are neither positive nor negative (CDC). The diagnostic value of TB blood tests is significant. The benefits of using blood tests in diagnosing TB range from the higher predictive value of TB compared with the TST (Diel et al 1164; Leung et al 839), to the fact that the results are available within 24 hours (CDC).

A TB blood test looks like an effective alternative to conventional skin tests in diagnosing TB. Brock et al write that the blood test is a reliable instrument of addressing operational inconsistencies in the TST (65). However, even blood tests are not without limitations. Numerous factors reduce the validity of the test results, from inappropriate collection and transportation of blood samples to incorrect interpretation of the test results (CDC). Brock et al suggest that, because approved blood tests for TB are based on purified protein derivative (PPD), they may have low efficiency in BCG-vaccinated populations (65). If that is the case, the use of CFP-10 and ESAT-6 antigens could increase the diagnostic agreement between the TST and QTF-TB tests (Brock et al 65). However, this hypothesis requires further analysis and validation. As of today, there is little doubt that blood and skin tests must be used as the two complementary measures in detecting TB disease. In the meantime, both tests leave significant room for further improvement in diagnosing TB.

Conclusion

Tuberculosis is rightly considered as one of the most problematic infectious diseases in many parts of the world. Today, blood and skin tests are used as the two complementary instruments of diagnosing TB disease. The Mantoux skin test is used to determine whether individuals have the Mycobacterium tuberculosis infection in their blood. The results are interpreted, based on the size of skin induration and the risks of getting infected with TB. The blood test measures the immune response to Mycobacterium tuberculosis. Blood tests demonstrate higher predictive value for developing active TB, compared with the TST. The latter is associated with the risks of producing false-positive and false-negative results. Simultaneously, the blood test may have low efficiency in BCG-vaccinated populations. Therefore, both tests leave sufficient room for further improvement in diagnosing TB disease.

Works Cited

Brock, I., K. Weldingh, Lillebaek, T., F. Follmann & P. Andersen. “Comparison of

Tuberculin Skin Test and New Specific Blood Test in Tuberculosis Contacts.” American Journal of Respiratory and Critical Care Medicine, 170 (2004): 65-69. Print.

CDC. “Tuberculosis: Testing & Diagnosis.” Centers for Disease Control and Prevention, n.d. Web. 01 February 2011.

Diel, R., R. Loddenkemper, K. Meywald-Walter, S. Niemann & A. Nienhaus. “Predictive

Value of a Whole Blood IFN-? Assay for the Development of Active Tuberculosis Disease after Recent Infection with Mycobacterium tuberculosis.” American Journal of Respiratory and Critical Care Medicine, 177 (2008): 1164-1170. Print.

Kowada, A., G.A. Deshpande, O. Takahashi, T. Shimbo & T. Fukui. “Cost-Effectiveness of

Interferon-Gamma Release Assay versus Chest X-Ray for Tuberculosis Screening of BCG-Vaccinated Elderly Populations.” Molecular Diagnosis and Therapy, 14.4 (2010): 229-36. Print.

Leung, C.C., W.C. Yam, W.W. Yew, P.L. Ho, C.M. Tam, W.S. Law et al. “T-Spot.TB

Outperforms Tuberculin Skin Test in Predicting Tuberculosis Disease.” American Journal of Respiratory and Critical Care Medicine, 182 (2010): 834-40. Print.