Evaluation of a quantitative enzyme-linked immunosorbent assay for feline leukemia virus p27 antigen and comparison to proviral DNA loads by real-time polymerase chain reaction.

Feline leukemia virus (FeLV) is an oncogenic retrovirus of cats. While higher viral RNA and proviral DNA loads have been correlated with progressive infections and disease, a similar correlation has been suggested for p27 antigen concentrations. This analytical study compared the results of a quantitative ELISA for p27 antigen with quantitative real-time PCR results for FeLV proviral DNA in patient samples. A significant positive correlation between copies of proviral DNA and the concentration of p27 antigen was identified (r = 0.761, P < 0.0001). Samples with high proviral DNA loads, at least 1 × 106 copies/mL of whole blood, typically had p27 antigen concentrations greater than 30 ng/mL in plasma. Samples with proviral DNA loads below this level all had concentrations of p27 antigen in plasma that were less than 10 ng/mL. Given this correlation, it is hypothesized that the concentration of p27 antigen at a given point in time may help to indicate the likelihood of a progressive or regressive infection similar to what has been demonstrated for proviral DNA loads.

Analytical validation of a reference laboratory ELISA for the detection of feline leukemia virus p27 antigen.

Feline leukemia virus (FeLV) is an oncogenic retrovirus of cats. Immunoassays for the p27 core protein of FeLV aid in the detection of FeLV infections. Commercial microtiter-plate ELISAs have rapid protocols and visual result interpretation, limiting their usefulness in high-throughput situations. The purpose of our study was to validate the PetChek FeLV 15 ELISA, which is designed for the reference laboratory, and incorporates sequential, orthogonal screening and confirmatory protocols. A cutoff for the screening assay was established with 100% accuracy using 309 feline samples (244 negative, 65 positive) defined by the combined results of FeLV PCR and an independent reference p27 antigen ELISA. Precision of the screening assay was measured using a panel of 3 samples (negative, low-positive, and high-positive). The intra-assay coefficient of variation (CV) was 3.9-7.9%; the inter-assay CV was 6.0-8.6%. For the confirmatory assay, the intra-assay CV was 3.0-4.7%, and the inter-assay CV was 7.4-9.7%. The analytical sensitivity for p27 antigen was 3.7 ng/mL for inactivated whole FeLV and 1.2 ng/mL for purified recombinant FeLV p27. Analytical specificity was demonstrated based on the absence of cross-reactivity to related retroviruses. No interference was observed for samples containing added bilirubin, hemoglobin, or lipids. Based on these results, the new high-throughput design of the PetChek FeLV 15 ELISA makes it suitable for use in reference laboratory settings and maintains overall analytical performance.

Analytical validation of a second-generation immunoassay for the quantification of N-terminal pro-B-type natriuretic peptide in canine blood.

N-terminal pro-B-type natriuretic peptide (NT-proBNP) has been shown to have clinical utility as a biomarker in dogs with heart disease. There were several limitations associated with early diagnostic assay formats including a limited dynamic range and the need for protease inhibitors to maintain sample stability. A second-generation Cardiopet® proBNP enzyme-linked immunosorbent assay (IDEXX Laboratories Inc., Westbrook, Maine) was developed to address these limitations, and the present study reports the results of the analytical method validation for the second-generation assay. Coefficients of variation for intra-assay, interassay, and total precision based on 8 samples ranged from 3.9% to 8.9%, 2.0% to 5.0%, and 5.5% to 10.6%, respectively. Analytical sensitivity was established at 102 pmol/l. Accuracy averaged 102.0% based on the serial dilutions of 5 high-dose canine samples. Bilirubin, lipids, and hemoglobin had no effect on results. Reproducibility across 3 unique assay lots was excellent with an average coefficient of determination (r (2)) of 0.99 and slope of 1.03. Both ethylenediamine tetra-acetic acid plasma and serum gave equivalent results at time of blood draw (slope = 1.02, r (2) = 0.89; n = 51) but NT-proBNP was more stable in plasma at 25°C with median half-life measured at 244 hr and 136 hr for plasma and serum, respectively. Plasma is the preferred sample type and is considered stable up to 48 hr at room temperature whereas serum should be frozen or refrigerated when submitted for testing. Results of this study validate the second-generation canine Cardiopet proBNP assay for accurate and precise measurement of NT-proBNP in routine sample types from canine patients.

Performance validation and method comparison of an in-clinic enzyme-linked immunosorbent assay for the detection of canine pancreatic lipase.

Diagnosis of pancreatitis is often difficult in dogs that present with acute vomiting, anorexia, and abdominal pain, as these clinical signs may occur with a variety of other illnesses. While quantitative reference laboratory methods specific for canine pancreatic lipase are available to aid in diagnosis, results are generally not available until the next day. The objective of the current study was to validate a semiquantitative in-clinic rapid test for the measurement of canine pancreas-specific lipase (cPL) and to compare its performance to the reference lab method. Comparison of the reference method for cPL to the in-clinic assay demonstrated 96-100% agreement for canine serum samples with normal levels of cPL and 88-92% agreement for samples with elevated levels of cPL. Common interfering substances such as bilirubin, lipids, or hemoglobin had no effect on assay performance. Both within-day and day-to-day variations ranged from 10% to 20% of the calculated cPL concentration, which demonstrated a high degree of precision for the in-clinic assay. Performance of 3 lots of the in-clinic assay with the same set of canine serum samples demonstrated high assay reproducibility, with interclass correlation coefficients of ≥0.93. Results of the in-clinic cPL assay, based on both visual and calculated cPL concentrations, were consistent throughout 15 months of storage. The in-clinic test provides immediate, semiquantitative results to supplement existing pancreatitis diagnostics at the time of acute illness. Because the reference and in-clinic methods are aligned, they can be used together as an immediate aid pet-side and as a fully quantitative follow-up test at the reference laboratory.