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

Abstract

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

Figure 1–

Comparison of interferon-γ-inducible protein (IP)-10 detected in dried plasma spots (DPS) and plasma. DPS and plasma samples from nil, antigen and mitogen samples from the training cohort comprising 60 patients with active TB and 59 healthy controls were compared. There was an excellent correlation between plasma and DPS IP-10 (Spearman r² = 0.95).

Figure 2–

Receiver operating characteristic curve analysis. Antigen-specific release of interferon (IFN)-γ and IFN-γ-induced protein (IP)-10 was compared in training set samples. IP-10 was detected in dried plasma spot (DPS) and plasma samples, IFN-γ in plasma using the QuantiFERON-TB Gold In-Tube (QFT-TB) ELISA. Areas under the curve were comparable, at 0.95, 0.94 and 0.89 for IP-10 plasma, IP-10 DPS and QFT-TB, respectively.