Evaluation of an in-practice test for feline coronavirus antibodies

Abstract

A commercially available in-practice test for feline coronavirus (FCoV) antibodies (FCoV Immunocomb, Biogal Galed Laboratories) was evaluated by comparison with the gold standard FCoV immunofluorescent antibody (IFA) test. One hundred and three serum or plasma samples were selected and tested: 70 were positive by both tests, 24 were negative by both tests. The in-practice test produced five false positive and four false negative results. The sensitivity of the in-practice test was 95% and the specificity was 83%. When the titres were compared it was found that the in-practice test results were significantly correlated with IFA titres but the degree of correlation was not likely to be clinically useful. The IFA titres of the four false negative samples were found to be low (less than 40) which suggests that even a cat with a false negative result is still unlikely to be excreting FCoV. A negative result with the in-practice assay is likely to be reliable for screening cats prior to entry into an FCoV-free cattery or stud. It would also be useful in the investigation of suspected FIP as most cats with this condition have high IFA titres of antibodies. A strong positive result would be useful in the diagnosis of FIP (in conjunction with other biochemical and cytological testing), but positive results would be of limited value in monitoring FCoV infection in healthy cats as the antibody titre could not be reliably compared with those obtained with IFA. All positive results obtained using the in-practice kit should be confirmed and titrated by IFA. The kit also appeared to work efficiently with ascites samples (n=6) but too few samples were analysed to draw firm conclusions.

Figure 1

Two Immunocomb teeth showing the three spots: positive and negative control spots and the sample one. Number 8 is positive, number 9 is negative.

Figure 2

The result dot is read by first setting the comb scale control (C3+) with the negative control spot, then matching the colour of the test spot to the comb scale. On this sample (white arrow), the result is 4.5.

Figure 3

A table comparing the positive and negative results obtained with 103 blood samples tested with both IFA and Immunocomb.

Figure 4

This graph shows the average Immunocomb result of the two readers and the logarithm of the titre obtained using the immunofluorescent antibody (IFA) test for each of the 103 blood samples.

Figure 5

This graph shows the average Immunocomb result of the two readers and the logarithm of the titre obtained using the immunofluorescent antibody (IFA) test for each of the six ascites samples.