Increased serum amyloid A and urinary kidney injury marker-1 concentrations in cats with pyelonephritis: a diagnostic accuracy study.

To investigate the discriminatory potential of serum amyloid A, neutrophil:lymphocyte ratio, monocyte:lymphocyte ratio and assess renal tubular injury in cats with pyelonephritis. Diagnostic accuracy study using samples from prospectively enrolled cats and stored material. Cats were classified according to diagnosis as pyelonephritis, ureteral obstruction, chronic kidney disease, subclinical bacteriuria, cystitis, or healthy. Serum amyloid A, neutrophil:lymphocyte ratio, monocyte:lymphocyte ratio and urinary kidney injury marker-1 were compared across groups. Receiver operating characteristic curve analysis assessed the performance of inflammatory biomarkers. Serum amyloid A and urinary kidney injury marker-1 were measured consecutively in a subset of cats with pyelonephritis. Samples from 71 cats (pyelonephritis n = 22, other urological conditions n = 35, healthy n = 14) were included. Median serum amyloid A concentration was significantly higher in cats with pyelonephritis, overlapping minimally with other groups. At a threshold of 49.1 mg/L, serum amyloid A showed a positive likelihood ratio of 32, sensitivity of 0.95 (95% CI: 0.74 to 0.99) and specificity of 0.97 (95% CI: 0.85 to 0.99), outperforming neutrophil:lymphocyte ratio. Serum amyloid A normalised in 12/14 cats post-treatment. Urinary kidney injury marker-1 was elevated in more than 50% of cats with pyelonephritis and ureteral obstruction, significantly higher than in healthy cats and those with subclinical bacteriuria or cystitis. Variation in urinary kidney injury marker-1 was observed during follow-up. At a threshold of 49 mg/L, serum amyloid A is a highly accurate biomarker for differentiating pyelonephritis from other urological conditions. Elevated urinary kidney injury marker-1 suggests frequent proximal tubular injury. Pyelonephritis is challenging to diagnose. Serum amyloid A can improve diagnostic precision and support timely treatment decisions. Urinary kidney injury marker-1 helps identify tubular injury; however, the clinical implications remain to be investigated.