Healthcare workers participating in annual tuberculosis screening had differences in interferon (IFN)-γ level depending on blood draw methods, according to study results published in the Annals of the American Thoracic Society.

Whole blood samples from healthcare workers in Texas who had a previous positive tuberculin skin test, a BCG vaccination, or immunosuppression were drawn into 3 QuantiFERON Gold (QFT-G) tubes, 2 QFT-P (Gold Plus) antigen tubes, and a lithium heparin blood tube (QFT-PT). The differences in IFN-γ level measured between methods was evaluated, and agreement was calculated between groups.

Of the 300 study participants with blood drawn, 299 had results from all 3 different methods. Of these 299 participants, 68% were female, 45% identified as Asian, 65% previously had the BCG vaccine, 79% were foreign-born, and 55% had a previous positive tuberculosis skin test. Overall, QFT-G and direct in-tube (QFT-PD) methods had the highest agreements, at 91.3%.

Using a conservative definition with an IFN-γ cutoff of both TB1 and TB2 ≥0.35 IU/mL for a positive QFT-P, 7% of participants with QFT-PD and 9% of participants with QFT-PT had positive results. The median IFN-γ measured in the Nil tube for the disagreeing QFT-PD and QFT-PT results was 0.18 and 0.13 IU/mL, respectively.

“Using the manufacturer’s interpretation for positive results, the overall percent agreement between QFT-G and QFT-P was high, regardless of blood collection method, but lowest between QFT-PD and QFT-PT,” the researchers wrote.

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They added, “The option of collecting blood into a single lithium heparin tube prior to blood transfer may be beneficial in lowering the proportion of indeterminate QFT in a clinical setting.”

Reference

Agarwal S, Nguyen DT, Lew JD, Graviss EA. Phlebotomy methods may affect QuantiFERON Gold Plus assay results [published online December 5, 2018]Ann Am Thorac Soc. doi:10.1513/AnnalsATS.201804-239RL