Relationship of spot urine oxalate to creatinine ratio and 24 hours urinary oxalate excretion in patients with urolithiasis

Background: The evaluation of 24 h urinary oxalate excretion is the gold standard for diagnosing hyperoxaluria in patients with recurrent urolithiasis. However, 24 h urine sample collection is cumbersome. Therefore we aim to see if oxalate to creatinine ratio in random urine sample can be used as an alternative. Materials and methods: A cross-sectional study was conducted at Section of Chemical Pathology, Department of Pathology and Laboratory Medicine Aga Khan University Karachi from 1st February to December 31, 2019. A total of 62 adult patients, 18-60 years of age with history of kidney stones presenting to the clinical laboratory for 24 h urine oxalate estimation were invited to participate in the study after informed consent. Clinical details were recorded on a structured questionnaire and patients were guided to submit 24 h urine and a random spot urine sample. Urinary oxalate was measured on Micro lab 300 using a kit based on oxalate oxidase principle by Trinity Biotech plc, Wicklow, Ireland following standard operating procedures. Urinary creatinine was measured on ADVIA 1800 by Siemens, US using kinetic Jaffe reaction according to the manufacturer's instructions. The data was analyzed on SPSS. Results: In a period of ten months, a total of 62 subjects were recruited; mean age was 32.4 2.6 years. Males were 49 (79.0%) and females were 13 (20.9%). Correlation was found to be (r = 0.289) by Spearman correlation (p value < 0.005). Taking 24 h urinary oxalate as gold standard the sensitivity, specificity, positive predictive value and negative predictive value of spot oxalate to creatinine ratio was 83.3%, 17.8%, 9.8% and 90.9% respectively. Conclusion: The random spot urine test cannot replace the 24 h urinary oxalate estimation in patients with urolithiasis.