Spot Urine Oxalate to Creatinine Ratio and 24 Hours Urinary Oxalate
Condition: Urolithiasis; Hyperoxaluria
MeSH terms: Urolithiasis; Urinary Calculi
Study type: Observational [Patient Registry]

Summary: 
      The evaluation of 24 hour urinary oxalate excretion is the gold standard for diagnosing
      Hyperoxaluria in patients with recurrent urolithiasis. The relationship of oxalate
      measurement between spot and 24 hour urine sample has not been studied in Pakistani
      population before. Thus, it is necessary to see if spot urine samples show good correlation
      with 24 hour urine samples in our population where the frequency of hyperoxaluria in patients
      with urolithiasis is reported to be 64.5%. Also, the various pre analytical issues associated
      with 24 hour urinary collection which may lead to the incorrect or misdiagnosis, need for
      duplicate testing consuming extra resources and man power. We therefore, in this study, want
      to see the correlation between 24 hour urinary oxalate and oxalate to creatinine ratio. The
      aim of our study is to determine the relationship between 24 hour urinary oxalate and spot
      urine oxalate to creatinine ratio and to identify if oxalate to creatinine ratio can be used
      as an alternative to 24 hour urinary oxalate
    

Detailed description: 
      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 31st
      December 2019. Adult patients with history of kidney stones presenting at the Clinical
      Laboratory for 24 hours urine oxalate estimation were included in the study. Debilitated or
      bed ridden patients and patients with renal failure or end stage renal disease, in whom
      urinary oxalate is no longer an indicator of disease status were excluded from the study.
      Study was approved by ethical review committee of Aga Khan University Hospital. Patients
      after informed consent, were guided to submit a spot urine sample. Clinical details were
      recorded on a structured questionnaire by the principal investigator.

      24 hour urine samples were collected in a jar having thymol, which is routinely added to
      limit bacterial growth and hence guard against citrate degradation. Spot urine samples were
      collected in red top container without any added preservatives. The specimen were transported
      to the laboratory within the 12 hours of collection of 24 hour urinary specimen.

      Volume of 24 hour urine was measured in liters. After mixing of the urinary sample to
      overcome effects of sedimentation, a 6 ml aliquot was made and HCl was added to avoid the
      precipitation of oxalate crystals in every sample. Specimen was stored at -20 C till further
      analysis.

      Urinary oxalate was measured on Micro lab 300 using a kit based on oxalate oxidase principle
      by Trinity Biotech plc. Wick low, Ireland following standard operating procedures. Urinary
      creatinine was measured on ADVIA 1800 by Siemens, US using kinetic Jaffe reaction. Both
      normal and abnormal quality control materials were run with every batch of oxalate and
      creatinine analysis in urine to validate the results. In addition, external proficiency
      testing of urinary oxalate was conducted by College of American Pathologists (CAP).

      The cut-offs for 24 hour oxalate excretion was 502 ųmol/24 hours for males and 353 ųmol/24
      hours for females. The cut-offs for spot oxalate: creatinine ratio was 33 ųmol/mmol for males
      and 45 ųmol/mmol for females.
    

Intervention name(s): Oxalate to creatinine ratio
Intervention type(s): Other
Intervention description(s): Comparsion between 24 hours urine oxalate and spot urine oxalate to creatinine ratio
Observational model: Case-Only
Time perspective: Cross-Sectional
Sampling method: Non-Probability Sample
Study population: 
        Adult patients with history of kidney stones presenting at the Clinical Laboratory for 24
        hours urine oxalate estimation.
      
Keywords: 24-hour urinary oxalate; Spot oxalate to creatinine ratio