Background and Aims: Assessing trends in renal function can be challenging when paired serum creatinine (SCr) levels are not available, particularly at the initial visit for evaluating renal function. The aim of this study was to validate the use of the spot urine creatinine-to-osmolality ratio (sUCr/Osm) as a surrogate indicator of urinary Cr 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 greater than or equal to 26.5 μmol/L (0.3 mg/dL) within the same duration. The threshold for sUCr/Osm was set at 7.07 when Cr is measured in μmol/L (or 0.08 when Cr is measured in mg/dL) to indicate values below the reference range.1 The comparative biomarker employed was urinary neutrophil gelatinase-associated lipocalin (UNGAL). Results: Of the 1570 participants, 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 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 with an estimated glomerular filtration rate 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 inferring “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 an SCr-based eGFR and sUCr/Osm.