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Should acidification of urine be performed before the analysis of calcium, phosphate and magnesium in the presence of crystals?

Author(s): Pratumvinit B, Reesukumal K, Wongkrajang P, Khejonnit V, Klinbua C, Dangneawnoi W

Publication: Clin Chim Acta, 2013, Vol. 426, Page 46-50

PubMed ID: 24012827 PubMed Review Paper? No

Purpose of Paper

Conclusion of Paper

Studies

1. Study Purpose

   This study investigated the effects of acidification of urine immediately or after one hour at room temperature on the presence of crystals and on the levels of calcium, magnesium, and phosphate. Sixty-eight leftover spot urine specimens from routine analysis containing crystals (3 uric acid crystals, 30 amorphous crystals, 23 calcium oxalate crystals, 5 triple phosphate crystals, 4 calcium oxalate and amorphous crystals, 1 uric acid and amorphous crystals, 1 uric acid and calcium oxalate crystals, and 1 calcium opalate and triple phosphate crystals) and 62 specimens without crystals were processed within 2 h or stored at 4˚C for up to 24 h. Specimens were divided into three groups that were: 1) left untreated, 2) immediately acidified with HCl to a pH of 1-2, or 3) stored for one hour at room temperature and then acidified with HCl to a pH of 1-2.  Levels of calcium, phosphate, magnesium, and creatinine were determined using a clinical chemistry analyzer.

   Summary of Findings:

   Specimens acidified immediately had a mean pH of 1.61 and those acidified after an hour at room temperature had a mean pH of 1.56. After acidification, 75 specimens contained uric acid crystal precipitates compared to five specimens prior to acidification. Compared to unacidified specimens, specimens acidified after one hour at room temperature had higher levels of calcium (12.00 versus 10.60, P<0.001) and phosphate (49.24 versus 48.41, P=0.008) and higher calcium creatinine ratios (0.114 versus 0.106, P<0.001), regardless of the presence of crystals and higher levels of magnesium (8.00 versus 7.70, P=0.006),phosphate creatinine (0.436 and 0.403, P<0.001), and magnesium creatinine (0.061 versus 0.057, P<0.001) in the presence of crystals. Compared to specimens acidified immediately, those acidified after one hour had higher levels of calcium (21.50 versus 21.30, P=0.025) and calcium creatinine ratio (0.162 versus 0.130, P=0.017) when crystals were present and lower levels of phosphate in specimens without crystals (43.90 versus 44.21, P=0.035), but no other effects of delayed acidification were noted.  Importantly, none of the differences observed between unacidified and acidified specimens exceeded the critical difference, thus none of the changes were considered to be clinically relevant. Levels of calcium, phosphate, and magnesium were strongly correlated between unacidified specimens and those acidified after one hour both in the absence of crystals (r=0.995, P<0.001; r=0.999 P<0.001; and r=0.964, P<0.001, respectively) and in the presence of crystals (r=0.880, P<0.001; r=0.990, P<0.001; and r=0.905, P<0.001, respectively). The authors note the higher correlation in levels of each analyte in the absence of crystals compared to those observed in the presence of crystals.

   Biospecimens

   • Bodily Fluid - Urine

   Preservative Types

   • Other Preservative

   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)
   Preaquisition	Biomarker level	Crystals present No crystals
   Storage	Time at room temperature	0 h 1 h
   Biospecimen Preservation	Type of fixation/preservation	Hydrochloric acid None (fresh)
   Biospecimen Aliquots and Components	pH	Neutral Low pH

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