665 - Lower Indeterminate Rates and Resolution by Retesting Using a Single Lithium-Heparin Tube Blood Collection Method for the QuantiFERON^®-TB Gold Plus (QFT^®-Plus)

Background: The QuantiFERON-TB Gold Plus (QFT-Plus) test is an assay for detecting a cell-mediated immune response to M. tuberculosis (MTB). The assay measures the in vitro quantitative IFN-γ responses to MTB or control antigens in an incubated blood sample. There are 2 options for QFT-Plus blood collection. One option is a lithium-heparin  transport tube with sample aliquots subsequently transferred to 4 QFT-Plus Blood Collection Tubes (1-tube QFT-Plus); the 2nd option is to directly collect the blood sample in 4 QFT-Plus  collection tubes (4-tube QFT-Plus). In this study, we compared the indeterminate (IND) rates by the 2 blood collection methods to assess which method was superior. 
 

Methods: For both blood collection methods, QFT-Plus ELISA testing was performed at various Quest Diagnostics sites as specified in the assay’s package insert. A retrospective data analysis of results for the above 2 blood processing methods was conducted. Also, we evaluated the rates of IND results in follow up blood collections.  Statistical analyses were performed by the proportion test. 
 

Results: In 2019, the IND result rate for greater than an 1.8 million 1-tube QFT-Plus draws was less than 1% whereas, the IND result rate for 0.3 million 4-tube draws was 4%   This difference was significant. The overall MTB positive rate was 7% for the 1-tube method and 6% for the 4-tube method.  Within a one-month interval following an initial blood collection event, 464 patients with an original IND result had a 2nd blood sample collected and tested. Only 35% of the 2nd blood collection events produced an IND result, with 52% of the 2nd sample results reporting as negative and 13% were positive. 

Conclusion: This study found that the 1-tube QFT-Plus collection method reduces the IND rates by 4-fold compared to the observed rate in the 4-tube process. Additionally, two thirds of patients with an initial IND result resolved to either a positive or a negative result when retested within 1 month.