MO377: Correlation Between 24-H Proteinuria and Spot Urine Albumin-Creatinine Ratio in Biopsy-Proven Glomerular Diseases
Authors: Díaz Enamorado Yunayka , Shabaka Amir , Dominguez Torres Patricia , Maria Cases Corona Clara , Landaluce Triska Eugenia , León-Póo Mariana , Maria Fernandez Juarez Gema 
Published: May 2022

Abstract

BACKGROUND AND AIMS
The gold standard for measurement of protein excretion is a 24-h urine collection. However, due to the cumbersome nature as well as the frequent errors in sample collection, it has been suggested that the measurement of albumin-creatinine ratio (ACR) in a spot urine sample could replace 24-h proteinuria as a valid method for the diagnosis and follow-up of renal disease, which is supported by the Kidney Disease: Improving Global Outcomes (KDIGO) guidelines. The aim of our study was to assess the correlation between 24-h proteinuria and spot urine ACR in patients with biopsy-proven glomerulonephritis,

METHOD
We identified patients with biopsy-proven glomerular diseases who were evaluated during their disease course at our center between 1 January 2005 and 31 December 2021, and included patients with paired 24 h and spot urine samples simultaneously collected at the same day. We excluded patients with interstitial or vascular causes for kidney disease. We contrasted correlation of ACR and 24 h proteinuria according to demographic data, glomerular filtration rate measured at the day of urine collection, and type of glomerular disease.

RESULTS
A total of 977 paired 24-hr proteinuria and spot urine ACR samples were collected at the same day from 370 patients and included in this study. The mean age of the selected population was 52.1 ± 17.7 years, with a median serum creatinine of 1.26 mg/dL (IQR: 0.94–1.74) and mean eGFR measured by CKD-EPI of 62.4 ± 30.1 mL/min. Median 24-h proteinuria was 1.48 g/day (IQR: 0.59–3.39), and the median ACR was 539 mg/g (IQR: 131–1569). We found an excellent correlation between ACR and 24-h proteinuria (Spearman's r = 0.86, P &lt; 0.001). This correlation was reproducible across all CKD stages, and remained strong even in patients with eGFR &lt;30 mL/min/1.73 m2 (Spearman's r = 0.85, P &lt; 0.001). On the other hand, after stratifying the cohort according to grade of proteinuria, we found that correlation was significantly weaker when proteinuria &gt;3 g/day (Spearman's r = 0.50, P &lt; 0.001) compared with lower grade proteinuria &lt;3 g/day (r = 0.79, P &lt; 0.001). We found no differences in correlation between ACR and 24-h proteinuria according to the type of glomerular disease, with excellent correlation across all glomerulonephritides.
ROC analysis showed that a ACR cutoff of &gt;250 mg/g was the best predictor for 24-h proteinuria &gt;1 g/day, with an area under the curve (AUC) of 0.933, sensitivity 91% and specificity 81%, while an ACR cutoff of &gt;800 mg/g best predicted a 24-hour proteinuria of &gt;3.5 g/day, with an AUC of 0.922, sensitivity of 92% and specificity of 76%.

CONCLUSION
There is an excellent correlation between ACR and 24-h proteinuria in patients with proteinuria &lt;3 g/day independent from eGFR and without significant variation according to the type of glomerular disease. In patients with proteinuria &gt;3 g/day, ACR correlates poorly with proteinuria. We suggest that patients with higher grade proteinuria should always be assessed with a 24-h urine sample protein excretion for therapeutic decisions and follow-up in glomerular diseases.