Sensitivity of a new commercial enzyme-linked immunospot assay (T SPOT-TB) for diagnosis of tuberculosis in clinical practice

Abstract

Diagnosis of active and latent tuberculosis (TB) remains a challenge; however, over the last few years, a new approach based on detecting Mycobacterium tuberculosis-specific T cells has shown much promise. In particular, there is substantial published evidence showing that the detection of ESAT-6- and CFP-10-specific T cells using the ex vivo enzyme-linked immunospot technique is a marked improvement over the existing tuberculin skin test. This technique, which detects gamma interferon-producing T cells, is now available as the commercial assay T SPOT-TB (Oxford Immunotec, Oxford, UK). In the present study, the usefulness of the T SPOT-TB test for diagnosis of TB in "real-world" clinical practice was investigated. Ninety patients of a southern German referral centre for TB with confirmed or suspected TB were randomly selected for this study. The results of the T SPOT-TB test were compared with the results of conventional diagnostic tools. The T SPOT-TB test detected 70 of 72 patients with pulmonary or extrapulmonary TB, indicating a sensitivity of 97.2% (95% confidence interval, 90.3-99.7). For 45 of these patients, tuberculin skin test (TST) results were also available. Only 40 (89%) of these 45 patients were positive in the TST compared to all 45 (100%) in the T SPOT-TB test (p=0.056). Among 12 of 90 patients for whom active TB disease was ruled out, the T SPOT-TB test was negative for 11 (92%), allowing the rapid exclusion of TB in patients suspected to have active TB disease. The T SPOT-TB test is a sensitive assay for detection of TB and represents a useful addition to the diagnostic algorithm available for detecting TB in low-incidence settings.

Use of a T-cell-based test for detection of tuberculosis infection among immunocompromised patients: Table 1—

“…The current authors' tentative conclusions are supported by the substantial body of literature demonstrating that the T-SPOT.TB TM test result correlates more closely with exposure [10][11][12][13][14], is more sensitive than the TST [7,[15][16][17][18] and is not prone to cross-reaction to BCG and most nontuberculous mycobacteria [11,[19][20].…”

“…This test was considered more useful than the QuantiFERON-TB Gold In Tube assay [6] since, at that time, only the T-SPOT.TB TM test had an internal positive control, and lymphocyte numbers and function were checked as part of the assay [7]. These two features were very important in the current setting because it was necessary to work with lymphocytes from people with underlying haematological diseases and who were undergoing chemotherapy, and thus it was necessary to know whether a negative result was a true negative or due to lack of function of T-cells.…”

Indeterminate test results of T-SPOTTM.TBperformed under routine field conditions

“…Of the four persistent ITRs, two were from patients aged o85 yrs and one from a patient aged 75-84 yrs. The higher rates of ITR observed in cold seasons could potentially have been a consequence of exceeding the recommended limits for temperature and duration of transport [8,24] However, ITR was not significantly associated with the distance travelled by the sample (,15 h) and so the T-SPOT TM .TB assay, therefore, appears robust even if the 8-h time limit is exceeded. The strong association with season suggests that the temperature at which the sample is stored during shipment is perhaps more important than the distance or duration of transport.…”

“…These tests with positive and nil internal controls are more specific than TST in diagnosing TB infection [3][4][5] and equally or more sensitive in patients with immune deficiencies. Nevertheless, indeterminate results have been reported for both tests with a frequency of 0-5.4% for the T-SPOT TM .TB test [6][7][8][9][10][11] and ,40% for the QuantiFERON1-TB Gold test [12][13][14][15][16][17]. Their occurrence seems to be associated with immunosuppression [10,18] and very young or very old age (patients aged ,5 or .80 yrs).…”

Clinical evaluation of QuantiFERON TB-2G test for immunocompromised patients

“…Finally, the decrease of IFN-c induced indeterminate results of the QFT-TB test because of the lower mitogen, ESAT-6 or CFP-10 QFT-TB levels. For these reasons, indeterminate results due to positive control failure seem to be less frequent with the T-SPOT.TB test, which detects individual Tcells producing IFN-c using an ELISPOT method, than the QFT-TB test [10,16,21]. This also applies to the use of the T-SPOT.TB in subgroups with impaired cellular immunity, with a recent report that HIV-infected subjects tested with the T-SPOT.TB showed only one (3%) indeterminate result, and that positive control responses were not adversely affected by CD4 counts [22].…”