Add-on testing: stability assessment of 63 biochemical analytes in centrifuged and capped samples stored at 16 °C.

OBJECTIVES: Integration of add-on testing in high-scale automated clinical laboratories constitute a valuable instrument not only for the clinicians and the general patient care, but also for the laboratory itself. Knowledge on sample quality and analytical stability upon storage is necessary to be able to offer add-on testing. The objectives of this study were to examine the analytical stability of 63 biochemical analytes in plasma and urine samples stored at 16 °C. METHODS: Samples were collected by professional laboratory technicians, analyzed at automated analyzers and stored in their primary, capped tube without separator for 10, 12, 16, 20 or 24 h at 16 °C. Stability was assessed by inspecting mean concentration of samples at baseline and examining if (A) mean concentration over time violated limits of bias, or if (B) individual sample concentrations violated limits of total error. RESULTS: The majority of the 63 analytes were stable for up to 24 h of storage. Few of the analytes were only suitable for add-on testing for 4, 6, 10, 12, 16 or 20 h of storage. One analyte, P-lactate dehydrogenase, was not found suitable for add-on testing when stored at 16 °C. CONCLUSIONS: Due to the increasing number of intelligent solutions for high-scale clinical laboratories, add-on testing has come to stay. Loss of stability could not be demonstrated for the majority of analytes after 10, 12, 16, 20 or 24 h of storage. This feature of analytical stability suggests that add-on testing is an acceptable tool for these analytes.

Urine soluble CD163 (sCD163) as biomarker in glomerulonephritis: stability, reference interval and diagnostic performance.

OBJECTIVES: Soluble (s) CD163 is a well-established macrophage biomarker, and recent data suggests urine sCD163 to reflect disease activity in crescentic glomerulonephritis (GN). Other types of GN may also be associated with glomerular inflammation but the potential usefulness of urine sCD163 as a general biomarker of GN remains unaddressed. METHODS: An in-house sCD163 enzyme-linked immunosorbent assay (ELISA) was validated for urinary use and compared to a frequently used commercial ELISA. The pre-analytical stability of urine sCD163 was assessed and a reference interval was established according to the CLSI guidelines using specimens from 253 healthy individuals. Urine samples from 64 patients with different types of renal disorders were also analysed. RESULTS: Urine sCD163 was highly stable during storage. An upper reference limit of 5.1 µg/L (1.9 µg/mmol, normalised to creatinine) was established using the in-house ELISA. Urine sCD163 was generally increased in GN patients (3.9 µg/mmol, p<0.0001, AUROC=0.97) and decreased upon treatment, but did not perform better than urine albumin (AUROC=1.00). Patients with proliferative GN had higher urine sCD163/albumin (p=0.0001) ratio. The commercial assay had a higher detection limit, and patient levels were 4-6 times lower than in the in-house assay. CONCLUSIONS: Urine sCD163 is a stable biomarker that can be measured with acceptable accuracy using our in-house ELISA. Its pre-analytical characteristics makes it a reliable biomarker and our findings point towards the use of urine sCD163 as a biomarker of specific subtypes of GN.