Comparing TSPOT assay results between an Elispot reader and manual counts

Serial T-SPOT.TB responses in Tanzanian adolescents: Transient, persistent and irregular conversions

Background

Prospective studies of interferon-gamma release assays (IGRA) on healthy subjects in tuberculosis-endemic regions have not examined the long-term variability of serial assays. This issue is relevant to the interpretation of tuberculosis (TB) vaccine trials based on prevention of infection.

Methods

T-SPOT.TB assays were performed manually on healthy adolescents during a tuberculosis vaccine trial in Tanzania at 5 intervals over 3 years. Assay results were defined as negative, positive, borderline or invalid. Subsequently, microtiter plates were analyzed by an automated reader to obtain quantitative counts of spot forming cells (SFCs) for the present analysis.

Results

3387 T-SPOT.TB samples were analyzed from 928 adolescents; manual and automated assay results were 97% concordant. Based on the quantitative results 143 (15%) participants were prevalent IGRA-positives at baseline, were ineligible for further study. Among the remaining IGRA-negative participants, the annual rate of IGRA conversion was 2·9%. Among 43 IGRA converters with repeat assays 12 (28%) were persistent converters, 16 (37%) were transient converters, and 15 (35%) comprised a new category defined as irregular converters (≥2 different subsequent results). ESAT-6 and CFP-10 responses were higher in prevalent than incident positives: 53 vs 36 for CFP-10 (p < 0·007); 44 vs 34 for ESAT-6 (p = 0·12).

Conclusions

Definitions of IGRA conversion, reversion, and persistence depend critically on the frequency of testing. Multiple shifts in categories among adolescents in a TB-endemic country may represent multiple infections, variable host responses in subclinical infection, or assay variation. These findings should to be considered in the design and interpretation of TB vaccine trials based on prevention of infection. Household contact studies could determine whether even transient IGRA conversion might represent exposure to an active case of M. tuberculosis disease.

A novel strategy for interpreting the T-SPOT.TB test results read by an ELISPOT plate imager

Background

The T-SPOT.TB can be read by an ELISPOT plate imager as an alternative to a labor-intensive and time-consuming manual reading, but its accuracy has not been sufficiently discussed to date.

Methods

1,423 test results obtained from manual reading using a microscope and an ELISPOT plate imager were compared. The agreement of qualitative test results was assessed using Cohen's kappa coefficient. The relationship of spot counts was studied using Bland-Altman analysis.

Results

The overall percent agreement of the qualitative test results was 95.43% with a kappa coefficient of 0.91. Positive test results with the maximum net spot count of 8 and borderline test results showed relatively high discordance. The agreement of spot counts in panel A, panel B, and nil control was good, and variability did not increase with higher spot counts. On the basis of study findings, a novel strategy for interpreting the test results by an ELISPOT plate imager was proposed.

Conclusions

To increase diagnostic accuracy, positive test results with the maximum net spot count of 8 and borderline test results should be manually confirmed. Our strategy could be a practical guide for laboratories to build their own strategies for interpreting the test results by an ELISPOT plate imager.