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The active ingredient in C-TST (formerly known as the ESAT6-CFP10 test, Anhui Zhifei Longcom, China) is an ESAT-6–CFP-10 fusion recombinant protein expressed in genetically modified E. coli. Each test dose of 0.1 mL contains 5 U of recombinant M. tuberculosis fusion protein, and auxiliary ingredients – disodium hydrogen phosphate, potassium dihydrogen phosphate, sodium chloride, phenol and polysorbate 80 (41).
Fig. 3.3. C-TST vials
Diaskintest (Generium, Russian Federation) is a recombinant protein produced by genetically modified culture of Escherichia coli BL21 (DE3)/pCFP-ESAT, diluted with sterile isotonic phosphate buffer solution, with a preservative (phenol). One dose (0.1 mL) of the product contains 0.2 μg of CFP-10–ESAT-6 recombinant protein, and excipients – disodium phosphate dihydrate, sodium chloride, potassium dihydrogen phosphate, polysorbate 80, phenol and water for injection (40).
Fig. 3.2. Diaskintest package and vial
Cy-Tb (formerly known as the C-Tb test, Serum Institute of India) contains a 1:1 ratio of two recombinant proteins of ESAT-6 and CFP-10 was previously produced by genetically modified Lactobacillus lactis, by Statens Serum Institute (Denmark). One single test dose of 0.1 mL contains 0.05 μg of rdESAT-6 and 0.05 μg of rCFP-10.
Fig. 3.1. Vial of Cy-Tb
The sensitivity of TST based on different size criteria of induration was established among people who had been treated for and had recovered from microbiologically confirmed TB (34). Of these, 98% had a reaction of 5 mm or more to TST, 90% had a reaction of 10 mm or more but only 60% had a reaction of 15 mm or more. On the other hand, reactions to TST due to BCG or nontuberculous mycobacteria are usually smaller than TST reactions due to true infection with M. tuberculosis (11, 12).
The original tuberculin material used by Mantoux in his first studies of tuberculin reactions was a highly heterogeneous mix of substances from killed M. tuberculosis(9). This so-called old tuberculin was replaced in 1941 by a standardized preparation of PPD from M. tuberculosis. A single standard lot of this material was produced by Florence Seibert, termed “PPDS” (32); since then, all newly produced tuberculin material has been produced using the same methods and tested against this standard, measuring induration in sensitized guinea pigs.
Skin testing is not advisable in people with a history of allergic reaction to TST or TBST. Allergic reactions to TST (PPD or equivalent), such as a generalized rash that occurs within the first 24 hours, are seen in less than 1% of recipients (26). If this has been well documented in the past, then it is wise to avoid repeating the test with the same tuberculin material. Currently, it is unclear whether use of an alternative tuberculin material would be safe.
TB infection tests should not be used for the diagnosis of pulmonary or extrapulmonary TB, nor should they be used for the diagnostic work-up of adults (including People with HIV) with presumed TB disease. TB infection tests should not be used for screening or to monitor the response to treatment for TB disease or TB infection.
TB infection testing may result in false negatives in individuals with certain viral illnesses (e.g. measles) or live virus vaccination (e.g. measles or mumps) within the preceding 30 days (23). This has been described with TST, but a similar effect with all TB infection tests is biologically plausible. Hence, it may be appropriate to delay the TB infection test for 30 days after infection or vaccination. Alternatively, a negative TB infection test may be repeated after 30 days.
If a prior positive TB infection test or TB treatment is documented, then repeat TB infection testing will not be useful and should not be done. Depending on the circumstances, the individual may be referred for further medical evaluation. However, if a prior positive result is self-reported and not documented, it is recommended to repeat the test, because studies have documented highly inaccurate self-reporting of prior skin test results (22).