Urinalysis Is More Specific for AKI than NGAL

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For early detection of acute kidney injury (AKI), urinalysis is a more specific test than urinary neutrophil gelatinase-associated lipocalin (NGAL), reports a study in Nephrology Dialysis Transplantation.

A rapid enzyme-linked immunosorbent assay (ELISA) for urinary NGAL underwent analytic validation by the use of random and 24-hour urine samples from 125 healthy volunteers. The results showed that NGAL levels were stable for up to 7 days, including ambient and frozen samples. The ELISA showed linear performance at concentrations from 0.24 to 10,000 ng/mL, with a quantitation limit of 0.24 ng/mL. Inter- and intra-assay precision were excellent, although the presence of white blood cells was associated with higher NGAL levels. The ninety-fifth percentile reference values were 65.0 ng/mL or less in women and 23.4 ng/mL or less in men.

In a clinical validation study, NGAL measurement and urinalysis were performed on samples from 363 emergency department patients who were admitted to the hospital. Urinary NGAL concentrations increased with AKI stage according to the Acute Kidney Injury Network criteria. However, the ELISA had only fair performance in differentiating absence of AKI versus stage 1, 2, or 3 AKI. Sensitivity and specificity were both 65 percent, with an area under the curve of 0.70. By comparison, urinalysis with microscopy offered excellent specificity of 91 percent but sensitivity of only 22 percent: area under the curve 0.57.

Urinary NGAL is a promising biomarker for earlier detection of AKI. The new ELISA reliably measures NGAL in clinical urine samples, although pyuria is a potential confounder.

Higher urinary NGAL is an indicator of AKI; its diagnostic performance is only fair, but it might be improved by excluding patients with prerenal causes of AKI. Meanwhile, microscopic urinalysis is a readily available and inexpensive test with high specificity for AKI [Schinstock CA, et al. Urinalysis is more specific and urinary neutrophil gelatinase-associated lipocalin is more sensitive for early detection of acute kidney injury. Nephrol Dial Transplant 2013; 28:1175−1185].

July 2013 (Vol. 5, Number 7)