Guidelines on the Prevention and Control of Tuberculosis in Ireland

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<strong>Guidelines</strong> on the Prevention and Control of Tuberculosis in Ireland 2010HSE/HPSCRecommendations for the use of IGRAIGRA use should be considered in conjunction with a clinical and public health risk assessment. If available,IGRA can be used for the diagnosis of LTBI in the following settings.Contact tracing (see chapter 8)• The TST (Mantoux test) should be used as the first line test for the diagnosis of LTBI in contactsof infectious TB cases and others considered to be at high risk of LTBI. Those with positive TSTresults should be considered for IGRA testing, if available (see figures 8.1 & 8.2)• IGRA may be considered on a case by case basis in adults and children as per the generalrecommendations in section 8.7. 60Pre-placement screening of HCWsIn new HCWs who are asymptomatic for TB and have a low pre-test probability of LTBI, IGRA, if available,can be used to confirm a positive TST result. Persons with a positive IGRA should be considered fortreatment of LTBI (see chapter 9).New entrant screeningAlthough the use of IGRA in screening new entrants has not clearly been demonstrated to date, the use ofIGRA can be considered:• As a confirmatory test in those individuals with a positive TST• In screening new entrants with concomitant conditions that increase the individual’s risk ofreactivation of LTBI (chapter 9).For individuals commencing on immunosuppressive therapy, i.e. tumour necrosis factor-α (TNF-α)antagonists.• IGRA, if available, can be used as an adjunct to screening in addition to a medical history, chestX-ray and TST.IGRA, if available, can be considered as the sole test for LTBI in the situation outlined below:• When screening large numbers of individuals as part of a public health investigation where logisticissues make repeated visits for sequential testing impractical. 54Recommendation:For contact tracing, the TST (Mantoux test) should be used as the first line test for thediagnosis of LTBI in contacts of infectious TB cases and others considered to be at high risk ofLTBI. Those with positive TST results should be considered for IGRA testing (see figures 8.1 &8.2). IGRA may be considered on a case by case basis in adults and children as per the generalrecommendations in section 8.7. 60IGRA performance in immunocompromised populationsThere are few studies on the sensitivity and specificity of IGRA in immunocompromised populations. TSTsensitivity is modest to poor in these populations. The sensitivity of T-SPOT.TB appears to be maintainedin immunocompromised individuals and appears to have a higher rate of positivity than TST. QuantiFeronstudies have not demonstrated this. 60 In the immunocompromised person (adult or child), the TST shouldbe the initial test used to detect LTBI. If the TST is positive, the person should be considered to have LTBI.However, in light of the known problem of false negative TST results in immunocompromised populations,a clinician still concerned about the possibility of LTBI in an immunocompromised person with an initialnegative TST result may perform an IGRA test. If the IGRA test is positive, the person might be consideredto have LTBI. If the IGRA result is indeterminate, the test should be repeated to rule out laboratory error.If the second test is negative, the clinician should suspect anergy and rely on the person’s history, clinical-19-
<strong>Guidelines</strong> on the Prevention and Control of Tuberculosis in Ireland 2010HSE/HPSCfeatures and other laboratory results to make a decision on the likelihood of LTBI. Either IGRA test may beused, however, there is evidence that the T-SPOT.TB assay may be more sensitive that the QuantiFeron testand this will be especially relevant for immunocompromised populations. 60While the approach of accepting either test result (TST or IGRA) as positive will improve the sensitivityof detecting LTBI in immunocompromised populations, there are no data supporting the efficacy ofpreventive therapy in TST negative but IGRA positive individuals. Thus the clinician must weigh thepotential benefit of detecting more persons with positive test results against the lack of evidence for thebenefit of preventive therapy in such persons.Potential boosting of IGRA by previous TSTPrevious guidelines by CDC state that the results of IGRA are not influenced by previous TST. 51 Studiesby Leyten et al 66 and Richeldi et al 67 report no boosting phenomenon following the evaluation of T-SPOT.TB assay. Some studies however, have reported boosting of the QuantiFERON-TB Gold® In –Tube assayresults when taken 6 to 8 weeks after a TST. 68;69 Although these studies have limitations, there are animalstudies suggesting that TST might boost subsequent measurements of interferon gamma. 70;71 This issuerequires further investigation. Due to these concerns, it is recommended by both the HPA and the PublicHealth Agency of Canada that the IGRA test should be undertaken at the time of reading the Mantouxresults. 54;60Serial testingThere are insufficient data to inform recommendations on serial testing with IGRA. Studies of serial testingof individuals with either LTBI or TB disease do not show a clear pattern. In some studies, IGRA responsesincreased, 72 decreased 73 or showed no change. 74 Further studies are needed.Diagnosis of active TB diseaseIGRA should not be used in the first instance for the diagnosis of active TB disease in either adults orchildren and should not replace the appropriate microbiological and molecular investigation.Culture remains the gold standard for the diagnosis of TB disease as it provides a definitive diagnosis andpermits the identification of drug resistance. IGRA have no benefits in known pulmonary TB cases withbacteriological/molecular confirmation.Recommendation:IGRA tests should not be used in the first instance for the diagnosis of active TB disease.Appropriate microbiological and molecular investigations remain the gold standard.However, in some patients (adults and children) with TB, it is not possible to isolate M. tuberculosis fromclinical specimens or to obtain clinical specimens, despite the individual having symptoms, signs and/or radiological changes consistent with the diagnosis of TB. In these circumstances, a positive IGRA mayincrease confidence in the diagnosis. In those with symptoms or signs compatible with but not indicativeof the diagnosis of TB, a positive IGRA test may suggest more strongly the possibility of a TB diagnosis. 54However, the final decision should be based on clinical judgment. 54 60 IGRA tests cannot distinguishbetween active TB and LTBI. 60Advantages of IGRA tests• It only requires one visit from the patient compared to two visits for a TST• IGRA demonstrate improved specificity over the TST i.e. the proportion identified as disease freeand the reduced cross-reactivity with BCG vaccine and most NTM means that persons are less-20-
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Guidelines on the Prevention and Control of Tuberculosis in Ireland

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