Test Data Sheet (Sample) - PDF

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					                                Test Data Sheet

Test code and name
TC0772 - IDEXX HerdChek BVD (ERNS) Ag/serum plus ELISA

Description
The test consists of an ELISA for the detection of Erns antigen of Bovine Viral
Diarrhoea Virus (BVDV) in bovine whole blood. Samples giving a positive
result are sent to VLA Penrith for typing by PCR.


Uses
The test is validated by VLA for use with bovine blood and can be used as a
diagnostic tool to determine whether an animal is infected with BVD virus. It
can be particularly valuable in determining the PI (persistent infection) status
of animals aged 3 weeks to 6 months. In cases where a negative result is
obtained for calves after intake of colostrum and where suspicion of BVDV
infection still exists, the animal should be re-bled and retested at an age of
more than 30 days. Samples may also be sent for BVDV PCR testing, for
confirmatory analysis.

Although not specifically validated by VLA, the manufacturer also claims this
test to be suitable for use with serum and tissues, including ear-notch tissue
samples.

Methodology
2ml of heparinised whole blood is usually required, but 1.0ml will suffice.
Samples are mixed with detection antibodies (biotinylated polyclonal
antibody), supplied in the kit, and added to the test plate. After incubation, the
plate is washed and a horseradish peroxidase (HRPO) conjugate is added.
After incubation, the plate is washed and a TMB chromogen substrate is
added to initiate colour development, which is stopped by adding an acid. The
test plate is then read on a spectrophotometer plate reader at a wavelength of
450nm with a reference wavelength of 620nm.
Results are expressed as a numerical value representing the sample-to-
negative ratio (S/N), derived from the OD values of the sample well and
negative control.

Specification and Validation
This ELISA has demonstrated considerably better performance with regards
to detecting positive samples in animal less than 6 months of age, where
maternal/colostral antibodies may mask the presence of the antigen,
compared to that of the Synbiotic’s ELISA method previously employed in
testing.

The manufacturer diagnostic sensitivity is quoted as being 100% with a 95%
confidence interval of 98.1%-100%.
The manufacturer diagnostic specificity is quoted as being 100% with a 95%
confidence interval of 97.6%-100%.
The test validation study carried out at VLA is contained within the test
validation document TV149.


Interpretation
Samples giving a sample/negative (S/N) result greater than 0.300 are
considered to be positive for BVD virus.
Samples giving a S/N result equal to or less than 0.300 are considered to be
negative for BVD virus.

A bovine sample giving a positive result can be said to be infected with BVDV.
A bovine sample giving a negative result should be treated with caution
particularly in younger animals where circulating maternal antibody from
colostrum intake may interfere with the detection of BVD Virus antigen.
Calves giving a negative result after intake of colostrum are recommended for
retesting when they reach more than 30 days of age, as recommended by the
manufacturer.

The validation data produced by VLA indicates that the IDEXX ELISA is
slightly less sensitive than the PCR in picking up PI animals that are less than
6 months old, using heparinised blood as a sample.


Further Information

This IDEXX test detects a different viral protein to that detected by the
Synbiotics antigen ELISA. The IDEXX test detects Erns, which is consistently
present in large amounts in whole blood, serum and tissues. This viral protein
is highly stable and is a secretory protein, as well as associated with viral
particles. For these reasons, this test is less affected by the presence of
maternal antibodies acquired through colostrum and therefore more suitable
for testing younger calves.

The old Symbiotics test detects the viral antigen NS2/3. This protein is only
associated with viral particles, so is present in less consistent amounts in
blood and tissues.

TDS/TC0772/Sonjia Kelly/September 2009