title: Should acidification of urine be performed before the analysis of calcium, phosphate and magnesium in the presence of crystals?
abstract: BACKGROUND
  Acidification of urine has been recommended before testing for calcium, phosphate, and magnesium. We investigated the necessity of pre-analytical acidification in both crystallized and non-crystallized urine samples.
  
  
  METHODS
  From 130 urine samples obtained via routine urine analysis, 65 (50%) samples were classified as non-crystallized. All samples were divided into three groups: untreated samples, acidified samples with HCl, and acidified samples after 1h room-temperature incubation. Urine samples were measured for calcium, phosphate, magnesium, and creatinine using Modular P800 and were examined for crystals using light microscopy.
  
  
  RESULTS
  In crystallized samples, acidified samples with 1h incubation had significantly higher Ca/Cr, P/Cr, and Mg/Cr than did untreated samples with mean differences of 0.04, 0.03, and 0.01 mg/mg, respectively (P<0.001). In acidified samples that were analyzed immediately, crystallized samples had lower calcium concentrations than those of acidified samples with 1h incubation and a mean difference of 0.21 mg/dl (P = 0.025). None of the sample differences which exceeded the critical difference of urinary Ca, P and Mg was observed.
  
  
  CONCLUSIONS
  Acidification of urine should be performed before the measurement of Ca, P, and Mg in the presence of urinary crystals. However, the lack of an acidification process does not result in a clinically significant change.
authors.affiliation: Department of Clinical Pathology, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand. Electronic address: busadee.pra@mahidol.ac.th.
authors.forename: Busadee
authors.initials: B
authors.lastname: Pratumvinit
authors.forename: Kanit
authors.initials: K
authors.lastname: Reesukumal
authors.forename: Preechaya
authors.initials: P
authors.lastname: Wongkrajang
authors.forename: Varanya
authors.initials: V
authors.lastname: Khejonnit
authors.forename: Cherdsak
authors.initials: C
authors.lastname: Klinbua
authors.forename: Weerapol
authors.initials: W
authors.lastname: Dangneawnoi
mesh_terms: D000143:Acids / Q000494:pharmacology; D002118:Calcium / Q000652:urine*; D003460:Crystallization; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000652:urine*; D010710:Phosphates / Q000652:urine*; D016482:Urinalysis / Q000379:methods*; D014556:Urine / Q000737:chemistry*
chemical_list: D000143:Acids; D010710:Phosphates; D008274:Magnesium; D002118:Calcium
keywords: Acidification; CD; CVs; Ca; Calcium; Cr; Crystal; Magnesium; Mg; P; Phosphate; SD; Urine; calcium; coefficients of variation; creatinine; critical difference; h; hour; magnesium; phosphate; standard deviation