Comparison of a point-of-care serum amyloid A analyzer frequently used in equine practice with 2 turbidimetric immunoassays used in human and veterinary medicine

Abstract

Rapid, accurate detection of serum amyloid A (SAA) is needed in equine practice. We validated a patient-side point-of-care (POC) assay (Stablelab; Zoetis) compared to the turbidimetric immunoassays LZ-SAA (TIA-Hum) and VET-SAA (TIA-Vet; both Eiken Chemical). Analytical performance was assessed at 3 different concentration ranges and with interferences. Inter-method comparison using 49 equine serum samples revealed a significant difference between median SAA results ( p < 0.0001), with the strongest bias between the POC and TIA-Vet (median 1,093 vs. 578 mg/L). The median SAA value obtained with the TIA-Hum method was 752 mg/L. Correlation between POC/TIA-Hum and between POC/TIA-Vet was fair (rs = 0.77 and 0.69) and excellent between both TIAs (rs = 0.93). Bias between POC/TIA-Hum, POC/TIA-Vet, and TIA-Hum/TIA-Vet was −56.7%, –80.9%, and −28.2%, respectively. POC intra- and inter-assay CVs (16.1–30% and 19.8–35.5%) were higher than TIA CVs (generally <12%). Bilirubin and hemoglobin had a negative bias on POC and TIA-Vet results (−16.6 to −45.6%); addition of intralipid yielded a positive bias (35.9–77.4%). The POC had good linearity of SAA concentrations up to 10,312 mg/L ( R2 = 0.92). A hook effect was present at SAA >3,000 mg/L for the POC assay. Equine serum SAA was stable over a median period of 2.5 y when stored at −80°C. Overall, there was excellent-to-moderate correlation between tests, but imprecision and hook effect of the POC, as well as bias between the methods, must be considered.

Use of serum amyloid A in equine medicine and surgery

“…StableLab (Zoetis) is available in Europe and the USA. This assay has been validated and compared with the most commonly used immunoturbidometric SAA assays (LZ‐SAA and VET‐SAA, both from Eiken Chemical Co.) 49,51 . Kiemle et al (2022) 51 demonstrated a high degree of constant and proportional concentration bias between the StableLab assay and the immunoturbidometric SAA assays.…”

“…This assay has been validated and compared with the most commonly used immunoturbidometric SAA assays (LZ‐SAA and VET‐SAA, both from Eiken Chemical Co.) 49,51 . Kiemle et al (2022) 51 demonstrated a high degree of constant and proportional concentration bias between the StableLab assay and the immunoturbidometric SAA assays. The StableLab assay had high intra‐ and interassay coefficient‐of‐variation, poor recovery rate, and a hook effect.…”

“…All these findings may severely limit the validity of results obtained with the assay, which users should be aware of. Based on the demonstration of the hook effect, the authors recommended that “it is advisable to perform repeat SAA measurements using the manufacturer's dilution protocol when clinical appearance of a horse and SAA results do not correlate and hence may represent falsely low concentrations secondary to a possible hook effect.” 51 …”

Acute phase reactants in nondomesticated mammals—A veterinary clinical pathology perspective

“…Several point‐of‐care (POC) devices are available specifically for the determination of SAA in cats and horses and CRP in dogs, and validation studies have been published for several of these systems in these species 77–80 . Generally, these POC tests for APRs have higher imprecision than the reference laboratory tests discussed in this article, and because assay antibodies may be different, there are also sometimes significant biases between methods 78,79 . Very high imprecision (exceeding, eg, the performance goals in Table 5) may limit clinical usefulness, and results and reference intervals are not interchangeable between methods.…”

“…[77][78][79][80] Generally, these POC tests for APRs have higher imprecision than the reference laboratory tests discussed in this article, and because assay antibodies may be different, there are also sometimes significant biases between methods. 78,79 Very high imprecision (exceeding, eg, the performance goals in Table 5) may limit clinical usefulness, and results and reference intervals are not interchangeable between methods. None of these devices have yet been investigated or validated in nondomesticated mammals.…”