Dried plasma spots in the diagnosis of TB: IP-10 release assay on filter paper

Interferon-γ release assays (IGRA) are probably the most accurate tests for detection of latent M.tuberculosis infection, but IGRAs are labour intensive and transport of samples over longer distances is difficult. IP-10 is expressed at 100-fold higher levels than IFN-γ, and IP-10 release assays have comparable performance to IGRA. The aim of this study was to explore the diagnostic potential of a novel IP-10 release assay based on dried plasma spots (DPS).IP-10 and IFN-γ was determined in plasma and in DPS by ELISA. Diagnostic algorithms for IP-10 plasma and DPS tests were developed on a training cohort comprising 60 TB patients and 59 controls. The diagnostic accuracy was assessed in a validation cohort comprising 78 TB patients and 98 healthy controls. Plasma was measured in Spain and DPS samples were sent to Denmark with conventional postal service for analysis.IP-10 was readily detectable in both plasma and DPS, and correlation was excellent (r(2)=0.95). QuantiFERON Gold-In-Tube (QFT-TB) and IP-10 in DPS and plasma rendered comparable sensitivity (78;82;84%), specificity (100;97;97%) and indeterminate rates (p>0.55).The DPS based IP-10 test has comparable diagnostic accuracy as the QFT-TB and samples can be sent with conventional mail over long distances for analysis without affecting the results.