Validation of a Point-of-Care Quantitative Equine IgG Turbidimetric Immunoassay and Comparison of IgG Concentrations Measured with Radial Immunodiffusion and a Point-of-Care IgG ELISA

BACKGROUND

Point-of-care (POC) diagnostic tests with good sensitivity and specificity are needed for diagnosing failure of transfer of passive immunity (FTPI) in foals. Turbidimetric immunoassays (TIA) have these characteristics and provide quantitative results. A commercially available TIA-based POC test (POC-TIA) has not been validated in horses.

OBJECTIVE

To validate a POC-TIA and compare results of POC-TIA, a POC-ELISA, and radial immunodiffusion (RID).

ANIMALS

Heparinized blood samples (n = 127) from 48 hospitalized foals (<12 hour to 48 days).

METHODS

Prospective validation study. IgG concentrations were measured using RID (gold standard), POC-TIA, and POC-ELISA. Agreement between assays was assessed using Bland-Altman analysis. Sensitivity and specificity were calculated using ROC curves. Inter- and intra-assay coefficients of variation (CVs) and linearity were evaluated for POC-TIA.

RESULTS

The mean bias (95% limits of agreement) between RID and POC-TIA was -4 (-185 to 176), 27 (-201 to 255), and 308 (-377 to 993) mg/dL for samples with IgG concentrations of <400, 400-800, and >800 mg/dL, respectively. Sensitivity and specificity at optimal cutoff were 94 and 100% for the POC-TIA and 94 and 100% for the POC-ELISA to detect IgG <400 mg/dL, and 85 and 87% (POC-TIA) and 69 and 79% (POC-ELISA) to detect IgG ≤800 mg/dL. Intra- and interassay CVs for POC-TIA ranged between 1.6-3.8 and 11.9-18.8%, respectively. Linearity of the dilution series was preserved (R² > 0.96).

CONCLUSIONS AND CLINICAL IMPORTANCE

The POC-TIA provided unambiguous results and had sufficient sensitivity, specificity, accuracy, and precision to be used as an alternative to other POC tests to assess FTPI in foals.

Tumor necrosis factor alpha (TNF-alpha) and sepsis: evidence for a role in host defense

Serum levels of TNF-alpha were evaluated in 29 patients with sepsis, using TNF-alpha sensitive L929 cells (sensitivity = 15 pg/ml). Blood samples were collected serially at the first 24-36 h of symptoms. Seventeen patients had severe underlying disease and 12 patients had mild or no underlying disease. Shock was present in 25 patients. Overall mortality was 62.1%. TNF-alpha was detected in nine patients (range: 57.7-3,169 pg/ml). There was a tendency to detect TNF-alpha in patients with mild or no underlying disease (p = 0.07). Detection of TNF-alpha was associated with survival (p = 0.0003) even when adjusted for severity of underlying disease (p = 0.005), shock (p = 0.0005), coagulation abnormality (p = 0.002) and immunosuppressive therapy (p = 0.005), using a bivariate analysis. In this investigation, detection of circulating TNF-alpha was predictive of good outcome in septic patients, suggesting a role for this cytokine in host-defense against this kind of infection.