New immunochromatographic rapid test for diagnosis of acute Puumala virus infection

Abstract

A new immunochromatographic rapid test, POC PUUMALA (Erilab Ltd., Kuopio, Finland), for detection of acute-phase Puumala virus (PUUV) infection was developed based on a highly purified baculovirus-expressed PUUV nucleocapsid protein antigen and lateral immunodiffusion techniques. After addition of sample (5 microl of serum, plasma, or fingertip blood) and buffer, PUUV-specific immunoglobulin M (IgM) antibodies, if present, together with the gold-conjugated anti-human IgM, formed a specific colored line in 5 min. The sensitivity and specificity of the test were evaluated with 200 serum samples and 30 fingertip blood samples. The reference method for the serum samples was a micro-capture enzyme immunoassay (EIA) for IgM and an immunofluorescence assay (IFA) for IgG antibodies. The analytical sensitivity and specificity of the rapid test were 100 and 99%, respectively, for unfrozen serum samples (n = 103; 12 PUUV IgM-positive samples). When freeze-thawed serum samples were used, the sensitivity and specificity were each 97.1% (n = 70; 35 PUUV IgM-positive samples). The specificity of the test was 96.2% for 27 serum samples with nonspecific IgM antibodies or rheumatoid factor (RF). The fingertip blood samples (n = 30) were negative, but they gave clear positive results when spiked with IgM-positive sera (n = 20). The results were in good agreement with the standard diagnostic methods. The rapid performance, the lack of need for refined laboratory equipment, and the high specificity with fresh serum and fingertip blood samples indicate that the developed POC PUUMALA rapid test is a useful tool for fast diagnosis of acute PUUV infection.

FIG. 1

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of baculovirus-expressed PUUV-N antigen (lane 1) and molecular mass markers (LMW; Bio-Rad Laboratories, Hercules, Calif.) (lane 2). In this experiment, ∼1 μg of the PUUV antigen was run into a 10% gel under reducing conditions. The gel was stained and destained with the Bio-Rad Coomassie brilliant blue staining system. As estimated from this gel, the level of purity in the antigen preparation is over 90%, and its molecular size is ∼50 kDa, which corresponds to the full-length nucleocapsid protein of PUUV.

FIG. 2

Typical positive and negative results with IgM-positive serum (row 1), IgM-negative serum (row 2), a fingertip blood sample spiked with IgM-positive serum (row 3), and an IgM-negative fingertip blood sample (row 4) in the POC PUUMALA rapid test. The visible control line (left) and visible test line (right) in the test window indicate a positive result. If only one red line appears in the control window, the test is negative.