Recommended Uses

PCR tests may be used as the primary test to detect organisms when cost is not a constraint. PCR tests also provide more rapid turn-around and/or greater sensitivity for some fastidious organisms such as Salmonella sp., Listeria monocytogenes, Lawsonia intracellularis, Brachyspira sp., Mycoplasma gallisepticum and Mycoplasma synoviae. Due to the elegant sensitivity of PCR tests, they are the test of choice when little of the target organism is expected in tissue and maximum sensitivity is required. PCR tests are the choice at Purdue ADDL for diseases that pose serious health threats to personnel since the first step in PCR tests inactivates most infectious organisms. These zoonotic agents include Chlamydia sp. and West Nile Virus. The elegant specificity of PCR allows for differentiation of closely related organisms and is the only test available at Purdue ADDL for differentiation of TGE and PRCV, of H1N1, H1N2 and H3N2 group A Influenza viruses and of strains of E. coli and Clostridium perfringens. PCR is also the best test for detecting certain infectious agents in fetal tissues where in-utero autolysis renders them undetectable by other tests. We recommend PCR tests for PRRS virus, Leptospira sp. and Chlamydia sp. when indicated by clinical history on fetuses. We feel that the PCR test for Leptospirosis the only reliable test to confirm infection with Leptospira sp.

Interpretation of PCR Test Results

All PCR tests performed at Purdue ADDL are based upon published protocols in peer-reviewed scientific literature and have been validated with known positive and negative samples. PCR detects specific segments of nucleic acid from the target organism. As such, a true positive test indicates the presence of specified nucleic acid from the target organism, not viable organisms. Unless specifically designed to do so, PCR tests do not differentiate vaccine from field strains of organisms. The high specificity of PCR tests means that false-positive tests due to improper test function is unlikely. False-positive PCR tests are most commonly a result of contamination of the sample with spurious target nucleic acid during sample collection or during the performance of the test .. Extreme care is taken at Purdue ADDL to prevent contamination during the performance of PCR tests. Contamination of samples is more likely during collection of samples under field conditions. Positive results must always be evaluated considering the methods used for sample collection. If disposable instruments were not used to collect samples, positive results should be cautiously interpreted. The high sensitivity of PCR also means that true-positive results occur frequently in samples from clinically normal animals in endemically infected populations. It is therefore important to interpret the significance of true-positive results in light of consistent clinical disease or lesions. Although PCR tests are very sensitive, false-negative results can occur due to the nature of the sample. For example, fecal samples contain enzyme inhibitors that may interfere with the enzymes used in PCR tests resulting in reduced sensitivity. Intestinal mucosa scraped from regions with gross lesions is always a better sample than feces for PCR testing. Sensitivity is also reduced by sub-optimal choice of tissues/samples. For example in cases of nervous listeriosis in ruminants, lesions and organisms are principally in certain cranial nerves and the medulla. In such cases, if forebrain or mid-brain is submitted instead of medulla for PCR testing, false-negative tests are common.