title: Discordance between urine pH measured by dipstick and pH meter: implications for methotrexate administration protocols.
abstract: OBJECTIVE
  To minimize toxicity of high-dose methotrexate (MTX) therapy, urinary alkalinization with frequent monitoring of urine pH is required. Urine pH is usually assessed by fast and convenient dipstick methods. When urine color interferes with dipstick measurement, as occurs in patients receiving MTX, alternative methods such as pH meters are used. Nursing staff caring for patients on high-dose MTX reported that urine pH results from dipstick and pH analyzers were often clinically discordant. As a result urine pH by dipstick and pH meter were compared in patients on high-dose MTX therapy and patients with normal-colored urine samples.
  
  
  METHODS
  We measured urine pH by dipstick and pH meter in 116 urine samples from 4 patients receiving high-dose MTX therapy, and in 50 normal-colored urine samples from 50 patients not on MTX therapy.
  
  
  RESULTS
  In patients on MTX therapy the mean (±standard deviation) bias between dipstick and pH meter urine pH was 0.7±0.4, compared to 0.4±0.3 in patients not on MTX. For patients on MTX clinical concordance between dipstick and pH meter urine results was poor around a clinical cut-off of pH 8.0. Of the 92 samples with a meter urine pH≤8.0, 72 had a discordant value by dipstick (pH>8).
  
  
  CONCLUSIONS
  Urine pH readings by dipstick and pH meter are not equivalent, and the bias between them is exacerbated in patients on MTX. Institutions with high-dose MTX therapy protocols should not alternate between dipstick and pH meter urine pH monitoring.
authors.affiliation: Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN 55905, USA.
authors.forename: Amy M
authors.initials: AM
authors.lastname: Wockenfus
authors.forename: Christopher D
authors.initials: CD
authors.lastname: Koch
authors.forename: Patricia M
authors.initials: PM
authors.lastname: Conlon
authors.forename: Linda D
authors.initials: LD
authors.lastname: Sorensen
authors.forename: Kari L
authors.initials: KL
authors.lastname: Cambern
authors.forename: Amy J
authors.initials: AJ
authors.lastname: Chihak
authors.forename: Julie A
authors.initials: JA
authors.lastname: Zmolek
authors.forename: Amy E
authors.initials: AE
authors.lastname: Petersen
authors.forename: Bradley E
authors.initials: BE
authors.lastname: Burns
authors.forename: John C
authors.initials: JC
authors.lastname: Lieske
authors.forename: Brad S
authors.initials: BS
authors.lastname: Karon
mesh_terms: D016022:Case-Control Studies; D003116:Color; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008727:Methotrexate / Q000008:administration & dosage / Q000009:adverse effects* / Q000652:urine; D016482:Urinalysis / Q000379:methods*
chemical_list: D008727:Methotrexate