Background and Aims: Assessing trends in renal function can be challenging when paired serum creatinine (SCr) levels are not available, especially at the initial visit for evaluating renal function. The aim of this study was to validate the use of a spot urine creatinine-to-osmolality ratio (sUCr/Osm) as a surrogate indicator of urinary creatinine excretion rate for inferring renal function trends.1 The objective is to create a flowchart that assists in determining appropriate outpatient management based on a single SCr value with sUCr/Osm. Method: The current study conducted a secondary analysis using the primary datasets provided by The Assessment, Serial Evaluation, and Subsequent Sequelae of Acute Kidney Injury (ASSESS-AKI) Study.2 The term “stable renal function” refers to the absence of an interval increase in SCr of greater than or equal to 26.5 μmol/L (0.3 mg/dL) from a visit three months earlier to the index visit. The definition of “unstable renal function” is the presence of an interval increase in SCr of 26.5 μmol/L (0.3 mg/dL) or more within the same duration. The cut-off value for sUCr/Osm was set at 7.07 when Cr is measured in μmol/L (0.08 when Cr is measured in mg/dL), which represents the 95% lower reference limit.1 A positive designation was given if the value of sUCr/Osm was not less than the 95% lower reference limit, and negative otherwise. The comparative test employed a tubular injury biomarker, urinary neutrophil gelatinase-associated lipocalin (UNGAL). The cut-off value for UNGAL was established at 100 ng/mL based on the maximum Youden's Index value. A positive result was defined as a UNGAL level less than or equal to 100 ng/mL, while a negative result was defined as a UNGAL level greater than 100 ng/mL. Results: Of all the participants (N=1,570), 596 (38.0%) were female sex. The age (mean ± standard deviation) was 64.6 ± 13.0 years, the body mass index (BMI) was 30.1 ± 7.7 kg/m2, and the SCr level was 102.5 ± 51.4 μmol/L (1.16 ± 0.58 mg/dL). The sensitivity of sUCr/Osm in identifying “stable renal function” for all individuals was 0.926 (95% confidence interval: 0.912-0.939), with a positive predictive value of 0.906 (0.891-0.921) and an accuracy of 0.845 (0.827-0.863). These values of sUCr/Osm were either better or comparable to those of UNGAL- 0.867 (0.849-0.884), 0.917 (0.902-0.932), and 0.808 (0.788-0.827), respectively. In individuals (N=521) with an estimated glomerular filtration rate (eGFR) of less than 60 cc/min/1.73m², sUCr/Osm also performed better or similarly to UNGAL. Table 1 provides direct comparisons between sUCr/Osm and UNGAL in identifying “stable renal function”. Conclusion: To assess renal function, sUCr/Osm may be effectively used to infer the trend. Figure 1 presents a flowchart for the corresponding management based on a single SCr-based eGFR and sUCr/Osm. 1Hsu SP, Chien CT. Reference Intervals of Spot Urine Creatinine-to-Osmolality Ratio as a Surrogate of Urinary Creatinine Excretion Rate. Dis Markers. 2022 Jul 25:2022:3549047. 2Acknowledgement: The Assessment, Serial Evaluation, and Subsequent Sequelae of Acute Kidney Injury (ASSESS-AKI) Study Investigators and supported by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). The data from the ASSESS-AKI study reported here were supplied by NIDDK Central Repository. This abstract was not prepared in collaboration with Investigators of the ASSESS-AKI study and does not necessarily reflect the opinions or views of the ASSESS-AKI study, NIDDK Central Repository, or NIDDK.