F A C U L T Y O F H E A L T H A N D M E D I C A L S C I E N C E S

U N I V E R S I T Y O F C O P E N H A G E N

Sensitivity and specificity of ELISA and PCR tests for Sensitivity and specificity of ELISA and PCR tests for identifying identifying Coxiella burnetiiCoxiella burnetii seropositive and shedder seropositive and shedder

cows using Bayesian analysiscows using Bayesian analysisS Paul1, N Toft1, JF Agger1, Ø Angen2, AB Christoffersen2, JS Agerholm1

1Dept. of Large Animal Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Denmark; 2National Veterinary Institute, Technical University of Denmark

To estimate sensitivity (Se) and specificity (Sp) of milk and blood antibody ELISA and milk PCR tests to determine Coxiella burnetii status in dairy cows

Bulk tank milk (BTM) samples of 12 herds were tested for the level of C. burnetii antibodies by ELISA

BTM antibody level 64 was used as cut-off to stratify herds into high and low antibody level groups

Milk ELISA Blood ELISA Milk PCR

454 lactating cows

230 lactating cows 224 lactating cows

Se and Sp of the tests were estimated using latent class modeling in Bayesian analysis

Conditional dependency (COC) between milk and blood ELISA tests was assumed and estimated

Milk PCR

Mil

k E

LIS

A

Blood ELISA

Fig.1: Comparison between the sensitivite and 1- specificity plots of milk PCR, milk ELISA and blood ELISA at recommended vs best fit cut-offs

Fig.2: A plot of the posterior mean prevalance estimates for two populations at different PCR cut-off values and at a cut-off of 30 for ELISAs

RESULTS

The best set of results was revealed at cut offs at 33 quantification cycles for PCR and at antibody level 30 for the ELISA tests (Fig.1)

Se and Sp of these tests at this cut-off combination:

Milk PCR Milk ELISA Blood ELISA

Se (95% PCI*) 0.87 (0.71-0.99) 0.57 (0.48-0.67) 0.42 (0.31-0.53)

Sp (95% PCI) 0.99 (0.95-1.00) 0.88 (0.84-0.93) 0.86 (0.81-0.91)

*Posterior credibility interval

PCR Se and Sp were significantly higher than those of both ELISAs

Se of milk ELISA was significantly higher than Se of blood ELISA

COC estimates confirmed the conditionally dependency of

ELISAs

Prevalence increased with increasing quantification cycles for PCR (Fig.2) indicating an increased number of cows with

low concentration of bacteria in milk

OBJECTIVE

MATERIALS AND METHODS

BACKGROUNDCoxiella burnetii infection is prevalent worldwide including DenmarkHigh plevence was observed in Danish dairy cattle in recent yearsDiagnosis is mainly based on the dectection of bacteria by PCR or dectection of anitibodies by immunological testsHowever, very little is know about the characteristics of these commonly used tests

CONCLUSIONS

PCR has higher Se and Sp than both ELISAs and therefore, is considered a better test for identifying shedder cows

However, high testing costs and risk of misclassifying intermittently shedding animals may be an argument for recommending PCR as screening test in a control program

454 lactating cows from the 12 herds were included in the study