Canine heterophilic antibodies as a source of false-positive B-type natriuretic peptide sandwich ELISA results

Validation of a dry-slide immunoassay for progesterone analysis in canine plasma in a clinical setting

BACKGROUND

The identification of canine ovulation is critical for successful breeding. Progesterone measurements are useful for identifying ovulation. Progesterone assays are also quantitative and easily accessed, making them valuable in veterinary practice.

OBJECTIVES

We aimed to validate a dry-slide immunoassay (DSI) for use in dogs, including a method comparison with the chemiluminescence assay (CLIA) and mass spectrometry.

METHODS

Twenty-nine bitches were prospectively recruited. Accuracy, precision, interference, and stability were evaluated. Method comparison between DSI and CLIA and mass spectrometry was conducted, and bias was calculated.

RESULTS

Repeatability was 8.0%-10.8%, and within-laboratory imprecision was 8.8%-11.1% for four concentration levels. Recovery under dilution was 61%-100%, and the method was linear to a concentration of ~50 nmol/L. Recovery after the addition of a high progesterone sample was 76%-83%. Minor changes were seen in one hemolytic and two lipemic samples. Storage at room temperature for 12-24 hours resulted in concentrations that were 57%-96% of the initial concentrations. For samples frozen at -80°C, the concentrations were reduced 17%-27%. There was a significant difference between results from the DSI and CLIA, and a proportional bias was seen when DSI was compared with mass spectrometry, where CLIA correlated better than DSI.

CONCLUSIONS

Precision and accuracy were acceptable. A proportional bias was seen between DSI and CLIA. A small amount of interference was seen with hemolysis and lipemia. Progesterone concentrations were decreased in samples stored at room temperature and -80°C. The results support the use of the DSI for ovulation timing but not for artificial insemination with frozen semen since progesterone concentrations might exceed the assay's linearity and precision limits.

Prevalence of heterophilic antibodies in serum samples from horses in an equine hospital, and elimination of interference using chicken IgY

BACKGROUND

Heterophilic antibodies in serum and plasma can interfere with mammalian antibodies in immunoassays and result in false test results, usually false positive. Although studies screening for heterophilic antibodies as well as elimination studies have been conducted in dogs and cats, knowledge of the presence of heterophilic antibodies in other species in veterinary medicine is limited. In this study, a 2-site sandwich-type interference assay that detects anti-mouse antibodies was used to detect heterophilic antibodies in a population of horses treated in an animal hospital.

RESULTS

A total of 194 serum samples from 127 individual horses were analyzed. There were 11/127 (8.7%) interference-positive horses, and these were analyzed in an assay exchanging the capture mouse IgG with chicken IgY. The positive samples were negative in the chicken IgY assay, indicating elimination of a possible interference, with the chicken-based assay. Four interference-positive samples were from geldings, and anti-Müllerian hormone (AMH) was analyzed from these samples. AMH concentrations were negative in these samples as expected in geldings, indicating that the heterophilic antibodies did not cause interference in the AMH assay.

CONCLUSION

The present study shows that there are heterophilic antibodies in horse serum samples like in samples from humans, dogs, and cats. The use of chicken-based reagents, such as chicken IgY, which do not cross-react with mammalian IgG, eliminates the effects of interfering antibodies in the samples. Equine heterophilic antibodies do not necessarily cause interference in commercial immunoassays.

Pre-existing canine anti-IgG antibodies: implications for immunotherapy, immunogenicity testing and immunoassay analysis

One of the most enigmatic features of humoral immunity is the prevalent presence of circulating autoantibodies against IgG. These autoantibodies consist of several subsets, including rheumatoid factors, anti-Fab/anti-F(ab′)₂-autoantibodies, and anti-idiotypic antibodies. Anti-IgG autoantibodies can impair the safety and efficacy of therapeutic antibodies and interfere with immunogenicity tests in clinical trials. They can also cross-react with allospecific IgG, presenting as heterophilic antibodies that interfere with diagnostic immunoassays. Owing to these factors, recent years have seen a resurgent interest in anti-IgG autoantibodies, but their underlying clinical significance, as well as biological roles and origins, remain opaque. Increased knowledge about canine anti-IgG autoantibodies could facilitate the development of canine immunotherapies and help in understanding and counteracting immunoassay interference. This study investigated the clinical significance and interconnection of heterophilic antibodies, anti-Fab, and anti-F(ab′)₂-autoantibodies in dogs. We performed a 2-year prospective follow-up of dogs with heterophilic antibodies and analyzed serum for anti-Fab and anti-F(ab′)₂-autoantibodies. Canine heterophilic antibodies can persist for at least 2 years in serum. A widespread occurrence of anti-Fab and anti-F(ab′)₂-autoantibodies was found, with reactivity to cryptic epitopes in the IgG hinge region and sporadic cross-reactivity with mouse IgG. Canine anti-Fab and anti-F(ab′)₂-autoantibodies are thus potential sources of clinical immunogenicity and immunoassay interference.

Characterization of canine anti-mouse antibodies highlights that multiple strategies are needed to combat immunoassay interference

Immunoassays are widely used for detection and quantification of analytes in biological samples, but are vulnerable to analytical errors caused by interfering sample substances. Of particular interest are endogenous anti-animal antibodies that may bind to the immunoassay antibodies and cause erroneous test results. This phenomenon is a hazard to patient safety in both human and veterinary medicine. Here, we demonstrate that anti-mouse antibodies in dogs bind selectively to different regions of the murine IgG molecule, cross-react with IgG from different species, and consist of all major antibody classes present in canine serum (IgA, IgG and IgM). The antibody characteristics varied among individuals and their prevalence differed between two dog breeds. The selective binding to different IgG regions suggests that the antibodies might not originate from immunization through exposure to mice or other species. These findings show that canine anti-mouse antibodies are highly heterogeneous in nature and therefore require a combination of strategies to be counteracted.

Investigation of interference from canine anti-mouse antibodies in hormone immunoassays

BACKGROUND

Canine anti-mouse antibodies are a potential source of immunoassay interference, but erroneous immunoassay results are not always easily identifiable. Anti-Müllerian hormone (AMH) is a marker for the presence of gonads in dogs, but elevated AMH concentrations in neutered dogs could also be caused by antibody interference. For other assays, a discrepant result obtained after antibody precipitation might indicate antibody interference.

OBJECTIVES

We aimed to evaluate if canine anti-mouse antibodies are a source of erroneous results in the AMH assay and if antibody precipitation with polyethylene glycol (PEG) is a useful tool for detecting antibody interference in a variety of immunoassays used in the veterinary clinical laboratory.

METHODS

Twenty-nine positive and 25 negative samples for anti-mouse antibodies were analyzed for AMH, canine total thyroxine (TT₄ ), canine thyroid-stimulating hormone (TSH) and progesterone before and after treatment with PEG. Results that differed by more than four SDs from the intra-assay coefficients of variation were considered discrepant. Elevated AMH concentrations in neutered dogs with anti-mouse antibodies and no visible gonads present were considered evidence of interference.

RESULTS

Evidence of antibody interference was found in two samples analyzed for AMH. The presence of anti-mouse antibodies did not lead to a higher proportion of discrepant results after PEG treatment for any of the immunoassays. The overall incidence of discrepant results for healthy controls was very high (73%).

CONCLUSIONS

Canine anti-mouse antibodies are a source of erroneous AMH results. Antibody precipitation with PEG is not a useful tool for detecting interference caused by such antibodies.

Prevalence of interfering antibodies in dogs and cats evaluated using a species-independent assay

BACKGROUND

Interfering antibodies in human serum and plasma are known to react with mammalian antibodies in immunoassays and cause false-positive test results. Although this phenomenon was recently shown in companion animals, knowledge regarding immunoassay interference in veterinary medicine is very limited.

OBJECTIVES

The aims of this study were to set up a species-independent immunoassay procedure to detect interference in serum samples, to screen for interference in a cross-section of canine and feline patient samples from an animal hospital, and to determine if the detected interference could be neutralized using an immunoassay based on nonmammalian reagents.

METHODS

A 2-site sandwich-type interference assay was set up using commercially available mouse reagents. A total of 369 serum samples from 320 dogs and 263 samples from 218 cats were analyzed using the interference assay. Multiple samples were submitted from 36 dogs and 39 cats. Nineteen samples identified as interference-positive were analyzed in an assay using chicken antibodies.

RESULTS

Interference was detected in samples from 28 dogs (9%) and 10 cats (5%) screened with the interference assay. Except for 1 cat, consistent results were obtained for all 75 dogs and cats that submitted more than 1 sample. The interference was eliminated when analyzed in the chicken-based assay (P < .001).

CONCLUSIONS

Substances with reactivity toward mouse IgG can be detected in serum samples from dog and cat patients using a 2-site interference assay. The detected substances are most likely interfering antibodies, possibly originating from immunization with other mammalian species.

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.

Detection by ELISA of C-terminal proBNP in plasma from cats with cardiomyopathy

The B-type natriuretic peptide prohormone (proBNP) is enzymatically cleaved into an inactive N-terminal peptide and a biologically active C-terminal peptide with many beneficial cardiorenal effects. The purpose of this study was to develop and test in cats with cardiomyopathy an immunoassay to quantify the concentrations of C-terminal proBNP in feline plasma. An anti-canine proBNP monoclonal antibody (UI-1021) was shown to have adequate binding affinity to proBNP 80-106 for use in a solid-phase immunoassay, and by epitope mapping to bind within positions 84-87 of feline proBNP. UI-1021 was paired with an affinity-purified rabbit polyclonal detection antibody to feline proBNP 100-106, in a sandwich ELISA with feline proBNP 80-106 standard. The linearity and analytical range and sensitivity of the assay were confirmed from 1.4 to 85 pmol/L. Spike recovery averaged 106.5% (95% confidence interval 78-135%). Within run and intra-assay coefficients of variation were <12%. A protease inhibitor mixture preserved proBNP 80-106 immunoreactivity for at least 5 days in plasma. Clinical verification of the ELISA was done using plasma from 13 cats with cardiomyopathy, whose C-terminal proBNP concentrations ranged from 1.7 to 78.8 pmol/L vs. <1.4-1.8 pmol/L in plasma from 18 healthy cats. Concentrations were found to be substantially lower than reported N-terminal proBNP concentrations, and similar to those of human heart failure patients where relative C-terminal BNP deficiencies have been proposed as contributory to the progression of the disease.

Falsely high B-type natriuretic peptide concentration in patients without heart failure attributed to AxSYM assay: case series of eight subjects

We report a case series of eight subjects complaining of non‐specific chest pain without heart failure, but with apparent high concentrations of plasma B‐type natriuretic peptide (BNP). No positive clinical characteristics were identified in physical examinations, cardiac imaging, laboratory findings, or pulmonary function tests. However, we observed unusually high BNP values when analysing blood samples of the patients using the AxSYM assay, and this was not supported by readings from Triage® or ADVIA Centaur® assays on the same samples, which showed BNP within the normal range. We believe that the possibility for false readings for high BNP levels in healthy individuals measured by AxSYM assay should be taken into account by physicians in clinical practice to avoid medical errors.

Paraneoplastic hypercalcemia in a dog with benign renal angiomyxoma

An 11-year-old, male, neutered crossbred Collie dog was presented for a history of polydipsia and polyuria. Diagnostic investigations revealed total and ionized hypercalcemia and an increased concentration of parathyroid hormone-related peptide. Abdominal ultrasonography and contrast-enhanced computed tomography of the abdomen revealed a right-sided, cystic-appearing renal mass. Cytological examination of ultrasound-guided aspirates of the mass revealed high numbers of spindle cells. The mass was removed en bloc via an ureteronephrectomy. Histopathological examination of the mass revealed neoplastic spindle cells in loosely packed and interlacing streams within a myxomatous stroma. Immunohistochemical examination with vimentin, von Willebrand Factor, and alpha-smooth muscle actin confirmed the mass to be a renal angiomyxoma. A minority of the neoplastic spindle cells showed positive cytoplasmic parathyroid hormone-related peptide immunostaining. The hypercalcemia resolved following surgery, and the parathyroid hormone-related peptide concentration returned to within the reference interval. The dog was no longer polydipsic or polyuric 1 year following surgery. The present report describes a previously unreported renal neoplasm causing paraneoplastic hypercalcemia and highlights the possibility of paraneoplastic hypercalcemia being caused by a benign neoplasm.