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Measurement of urine calcium, magnesium and phosphate: Preanalytical considerations in the elimination of the acidification step.

Author(s): Unsihuay D, Reust J, Bolte K, Poventud-Fuentes I, Patel K

Publication: Clin Chim Acta, 2025, Vol. 568, Page 120128

PubMed ID: 39788343 PubMed Review Paper? No

Purpose of Paper

Conclusion of Paper

Studies

1. Study Purpose

   This study investigated the effects of initial pH, specific gravity and turbidity on bias in the quantification of calcium (Ca), magnesium (Mg) and phosphate (Phos) in urine specimens that were not acidified relative to those that were acidified (control). Additionally, the effects of centrifugation on bias in unacidified specimens and the effects of centrifugation on the measurement of Ca in specimens with CaOx crystals were investigated in a subset of specimens.  Leftover spot urine specimens from 107 patients (median age of patients was 9 years, range 5 days-46 years) were stored at 4°C for < 24 h before analysis. Urine pH was measured using a benchtop pH meter, and turbidity and specific gravity were measured using an iQ200 Series Urine Microscopy Analyzer. A 5 mL aliquot of each specimen was acidified to a pH of 3-4 with 6 N HCl and stored for 10 min before quantification of Ca, Mg and Phos in both the acidified (control) and unacidified specimen using a VITROS 4600 Chemistry system. A total of 13, 10 and 5 specimens were excluded from analysis of Ca, Mg and Phos, respectively, because values were below the limit of quantification. To investigate the effects of turbidity, aliquots of 37 urine specimens (9 clear, 5 hazy, 13 turbid and 10 extremely turbid) were centrifuged at 2,300 g for 5 min prior to acidification (or not) and quantification of Ca, Mg and Phos. To determine how centrifugation affects Ca levels in urine with CaOx crystals aliquots of 5 urine specimens with CaOx crystals (2 many, 2 several and 1 occasional) were centrifuged at 2,300 g for 5 min prior to acidification and Ca levels were compared to uncentrifuged controls.  Acceptability limits were defined as 4% or ± 0.5 mg/dL for Ca, 12.5% or ± 0.1 mg/dL for Mg, and 5% or ± 0.15 mg/dL based on 50% of the total allowable error.

   Summary of Findings:

   The pH of the unacidified urine ranged from 5-9 and the specific gravity from 1.00 to 1.036.  A total of 27, 6 and 17 specimens showed a bias in Ca, Mg and Phos measurements in the unacidified specimen that exceeded the acceptability limits. Regression analysis showed a mean bias of -1.36 in Ca, -0.64 in Mg and -0.338 in Phos levels in the unacidified specimen. Bias in Ca, Mg, and Phos measurements did not tend to increase at either end of the pH or specific gravity range. However, 13 of 24 turbid and 11 of 13 extremely turbid specimens had unacceptable biases in Ca, Mg and Phos compared with only 5 of 59 clear or hazy specimens. The mean bias in unacidified specimens was reduced for Ca and Mg in the 37 specimens that were centrifuged (-0.11 for Ca, -0.17 for Mg and 1.06 for Phos), and correspondingly there were fewer specimens with unacceptable bias (3 in Ca, 0 in Mg and 1 in Phos). In the 5 specimens with CaOx crystals, there was no significant effect of centrifugation prior to or after acidification on Ca relative to specimens that were acidified and uncentrifuged.

   Biospecimens

   • Bodily Fluid - Urine

   Preservative Types

   • None (Fresh)

   Diagnoses:

   • Not specified

   Platform:

   Analyte	Technology Platform
   Small molecule	Clinical chemistry/auto analyzer
   Electrolyte/Metal	Clinical chemistry/auto analyzer

   Pre-analytical Factors:

   Classification	Pre-analytical Factor	Value(s)
   Biospecimen Aliquots and Components	Centrifugation	Centrifuged Not centrifuged
   Biospecimen Aliquots and Components	pH	Unadjusted Low pH

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