Chronic kidney disease (CKD) is a common disorder and cause of death in cats. In the classification proposed by the International Renal Interest Society (IRIS), stage 1 and 2 CKD are difficult to diagnose accurately using markers, in comparison with normal controls. We recently described a simple and highly reproducible tandem mass tag labelling method for identifying potential disease-marker candidates among low-abundance plasma proteins. We identify novel plasma biomarkers that are potentially superior to classical markers for early and specific detection of CKD stage 1 cats. In the current study, Plasma samples were obtained from 43 normal controls and from 19 cats with CKD (stage 1 and 2). To identify new serum biomarkers for CKD, six plasma samples (two normal control samples, two CKD stage 1 samples and two CKD stage 2 samples) were extracted with LCA lectin magnetic beads, differentially labelled with TMT, digested with trypsin and subjected to analysis using LC-MS/MS. Apolipoprotein A-IV was identified as a protein with lower levels in CKD stage 1 cats. An ELISA of plasma sialylated Apolipoprotein A-IV showed within-run (6.0-8.2%) and between-day (9.1–10.4%) reproducibility. Plasma sialylated Apolipoprotein A-IV levels measured with this assay were significantly greater in normal controls than in CKD stage 1 cats (0.17±0.06 vs. 0.07±0.01 AU/mL, p<0.01). These results indicate that sialylated Apolipoprotein A-IV may be useful as a complementary marker with plasma creatinine, BUN and SDMA for detection of CKD stage 1 cats.