Reliability of Spot Urine Protein-to-Creatinine and Urine Albumin-to-Creatinine Ratios in Acute Kidney Injury: A Cohort Study.

RATIONALE &

OBJECTIVE

Spot urine protein-to-creatinine (UPCR) and albumin-to-creatinine (UACR) ratios are common surrogates for 24-hour excretion, but their validity during acute kidney injury (AKI) is uncertain because kidney function is not at steady state. This study assessed the validity of these spot ratios during active AKI compared to after its resolution.

STUDY

DESIGN

A prospective paired cohort study.

SETTING &

PARTICIPANTS

One hundred hospitalized adult patients with AKI at a single center in Thailand.

EXPOSURE: Active phase of AKI was the period between diagnosis and resolution, which was defined as the return of serum creatinine to baseline or a value ≥50% below the peak value, sustained for ≥48 hours.

OUTCOMES: The agreement and reliability between protein-to-creatine and albumin-to-creatinine ratios in spot urine samples versus in 24-hour urine collections.

ANALYTICAL APPROACH: Agreement was evaluated using Lin's Concordance Correlation Coefficient (CCC) and Bland-Altman analysis to quantify systematic bias and random error. Limits of Agreement (LoA) were defined as the range within which 95% of the differences between the two measurement methods were expected to fall.

RESULTS

Spot ratios differed from 24-hour collections during AKI (CCC: UACR 0.81; UPCR 0.68) and became more reliable after recovery, (CCC: UACR 0.89; UPCR 0.83) (p=0.001). Limits of agreement were wide during AKI (UACR 0.22 to 8.37; UPCR 0.31 to 7.99) and narrowed significantly upon resolution (UACR 0.40 to 5.48; UPCR 0.47 to 4.91). Spot ratios systematically overestimated the 24-hour collections by 36% to 56% both during and after resolution of AKI. The unreliability of the spot urine values was most pronounced in patients with preserved baseline kidney function, severe AKI, and hemodynamic etiologies.

LIMITATIONS: Defining resolution via serum creatinine may not reflect complete resolution of AKI. The exclusively hospitalized cohort may limit generalizability to milder AKI occurring in the outpatient setting.

CONCLUSIONS

Spot urine ratios exhibit significant imprecision during active AKI compared to following its resolution. Clinicians should exercise caution when using these data to establish baseline proteinuria or monitor treatment response during AKI.