Willeke P.J. Franken1, Steven Thijsen2, Ron Wolterbeek3, John J.M. Bouwman2 , Hanane el Bannoudi1, Sandra V. Kik4,5, Jaap T. van Dissel1, Sandra M. Arend1
T-SPOT.TB is a specific assay for the diagnosis of tuberculosis. The assay needs to be performed with freshly isolated cells and interpretation requires training. T-SPOT.TB has been used in various clinical-epidemiological settings, but so far no studies evaluated the effect of inter-observer variation in test reading.
To evaluate variation between different observers in reading T-SPOT.TB results.
Materials & Methods
The study was nested within an ongoing cohort study, in which part of the T-SPOT.TB had been performed with frozen material. Culture plates were read visually by four different observers from two laboratories, and by two automated readers.
Of 313 T-SPOT.TB assays, 235 were performed with fresh and 78 with frozen cells. No significant difference was found between results obtained with fresh or frozen cells. The percentage of positive results varied between readers by maximally 15%; 5/6 raters were within a 6% difference in positive results. Analysis of the observed inter-rater differences showed that some individuals systematically counted more spots. Because test interpretation includes subtraction of background values, this systematic variance had little influence on inter-individual differences.
The test result as positive or negative varied between independent raters, mainly due to samples with values around the cut-off. This warrants further study regarding determinants affecting the reading of T-SPOT.TB.