Preservation of urine specimens for metabolic evaluation of recurrent urinary stone formers.
Author(s): Šálek T, Musil P, Vermeersch P, Marrington R, Dikmen ZG, Poláchová R, Kipman U, Kouri TT, Cadamuro J
Publication: Clin Chem Lab Med, 2024, Vol. , Page
PubMed ID: 39072394 PubMed Review Paper? No
Purpose of Paper
This paper investigated the effects of urine acidification on levels of urinary stone-related metabolites/electrolytes by comparing magnesium, sodium, potassium, creatinine, oxalate, citrate, chloride, and uric acid levels in non-acidified urine and urine that was acidified before (day 0) or after 2 or 7 days at room temperature from patients with recurrent urinary calcium stones.
Conclusion of Paper
The mean variability between replicate unstored specimens was <3% for most analytes, although variability was <4% for oxalate and citrate levels. In non-acidified specimens on day 0, creatinine concentrations were strongly correlated with concentrations of magnesium, inorganic phosphate, uric acid, and oxalate and were modestly correlated with citrate levels. As expected, the concentration of chloride was higher in specimens that were acidified with hydrochloric acid. Although, urinary levels of sodium, potassium, chloride, and creatinine were unaffected by room temperature storage (<10% change from day 0 in non-acidified specimens), storage of non-acidified specimens led to failures (>10% change relative to day 0 in non-acidified specimens) in calcium after only 2 days and failures in magnesium, calcium, phosphate, and uric acid after 7 days; however, the number of failures decreased slightly when urine specimens were acidified after the storage (2 or 7 days). When specimens were acidified prior to storage, no failures were observed for calcium and phosphate, and fewer failures were observed for magnesium, but a high number of failures were observed for uric acid after 2 or 7 days of storage. Oxalate and citrate required acidification for analysis and, consequently, were normalized to the day 0 acidified specimen. When urine was acidified after the storage period but before analysis, a large number of failures (>10% change relative to urine specimens acidified at measurement on day 0) in oxalate or citrate were observed on day 2 and 7, but these were slightly reduced when the specimen was acidified prior to storage. Based on their findings, the authors recommend that urine must have a pH >6.5 for uric acid and chloride analysis, but urinary pH should be adjusted to <2 for magnesium, sodium, potassium, creatinine, oxalate and citrate analysis.
Studies
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Study Purpose
This study investigated the effects of urine acidification on levels of urinary stone-related metabolites/electrolytes by comparing magnesium, sodium, potassium, creatinine, oxalate, citrate, chloride, and uric acid levels in non-acidified urine and urine that was acidified before (day 0) or after 2 or 7 days at room temperature from patients with recurrent urinary calcium stones. First morning urine was self-collected at home by twenty patients (8 women and 12 men, 18-77 years of age) with recurrent urinary calcium stones (ten with calcium oxalate monohydrate stones, four patients with calcium oxalate dihydrate stones, and six with mixed calcium oxalate, calcium apatite, or unknown calcium stones) and transported to the hospital within 2 h of voiding. Urine was immediately analyzed and then split into aliquots for storage at room temperature for 0, 2 or 7 days. Aliquots either remained unacidified or were acidified by the addition of 5 μL 6M hydrochloric acid per mL of urine before or after storage. Particles and cells were analyzed by iQ200 assay and microscopy. Prior to all other analysis, particles were solubilized to the extent possible through incubation at 56°C for 30 min. Levels of calcium, magnesium, inorganic phosphate, uric acid, sodium, potassium, chloride, and creatinine were measured in duplicate using an Abbott Architect ci16200 analyzer. Prior to quantification of oxalate and citrate by capillary electrophoresis, 35 μL of 7M phosphoric acid was added to the urine.
Summary of Findings:
The mean variability between replicate unstored specimens was <3% for most analytes, although variability was <4% for oxalate and citrate levels. In non-acidified specimens on day 0, creatinine concentrations were strongly correlated with concentrations of magnesium (ρ =0.729, p <0.01), inorganic phosphate (ρ =0.839, p <0.01), uric acid (ρ =0.868, p <0.01), and oxalate (ρ =0.770, p <0.01), modestly correlated with citrate (ρ =0.613 p <0.01), and not significantly correlated with calcium (ρ =0.408, p =0.075). As expected, chloride concentration was higher at all timepoints in specimens that were acidified with hydrochloric acid. Levels of sodium, potassium, chloride, and creatinine were unaffected by room temperature storage for up to 7 days, regardless of acidification(<10% change from day 0 in non-acidified specimens). In non-acidified urine stored at room temperature, failures (a >10% difference from day 0 in non-acidified specimens) were only observed after 2 days of room temperature storage for calcium (two specimens, one with a >20% change and hematuria); however, failures occurred for magnesium (3 specimens all with >20% change), calcium (6 specimens, 5 with >20% change), phosphate (4 specimens, 3 with >20% change) and uric acid (2 specimens, 1 with >20% change) after 7 days. When specimens were acidified after the storage (2 or 7 days) but before analysis, slightly fewer failures (>10% relative to never acidified specimens on day 0) were observed: calcium failed in one specimen after 2 days (the one with hematuria) and failures were observed in magnesium (4 specimens, one with >20% change), calcium (5 specimens, all with >20% change), phosphate (3 specimens, 1 with >20% change) and uric acid (2 specimens, both with >20% change) after 7 days. Additionally, in specimens acidified after storage (as required for measurement of oxalate and citrate), failures (>10% change relative to acidified at measurement on day 0) were observed on day 2 for oxalate (7 specimens, 2 with >20% change) and citrate (5 specimens, 1 with >20%change), with more observed on day 7 (14 specimens, 8 with >20% change in oxalate and 9 specimens, 7 with >20% change in citrate). When specimens were acidified prior to storage, failures (>10% relative to day 0 and never acidified for all except oxalate and citrate) were observed for uric acid (5 specimens, 4 with >20% change), oxalate (6 specimens, 3 with >20% change) and citrate (2 specimens) after 2 days and for magnesium (1 specimen), uric acid (7 specimens, 6 with >20% change), oxalate (16 specimens, 5 with >20% change) and citrate (6 specimens) after 7 days. Based on their findings, the authors recommend that urine must have a pH >6.5 for uric acid and chloride analysis, but urinary pH should be adjusted to <2 for magnesium, sodium, potassium, creatinine, oxalate and citrate analysis.
Biospecimens
Preservative Types
- None (Fresh)
Diagnoses:
- Other diagnoses
Platform:
Analyte Technology Platform Electrolyte/Metal Clinical chemistry/auto analyzer Cell count/volume Hematology/ auto analyzer Small molecule Capillary electrophoresis-MS Small molecule Clinical chemistry/auto analyzer Pre-analytical Factors:
Classification Pre-analytical Factor Value(s) Biospecimen Preservation Type of fixation/preservation Hydrochloric acid
None (fresh)
Storage Storage conditions Not acidified
Acidified before storage
Acidified after storage
Biospecimen Aliquots and Components pH Low pH
Unadjusted
Storage Time at room temperature 0 days
2 days
7 days