title: LC-MS/MS Measurement of Parathyroid Hormone-Related Peptide.
abstract: BACKGROUND
  Parathyroid hormone-related peptide (PTHrP) is involved in activating pathways, allowing tumor cells to form bone metastases. Measurement of PTHrP is used for the diagnosis and clinical management of patients suspected of hypercalcemia of malignancy. We developed an LC-MS/MS method for measuring PTHrP, established sex-specific reference intervals, and assessed the method's performance.
  
  
  METHODS
  PTHrP was enriched from plasma samples with rabbit polyclonal anti-PTHrP antibody conjugated to magnetic beads. Enriched PTHrP was digested with trypsin, and PTHrP-specific tryptic peptide was analyzed with 2-dimensional LC-MS/MS in multiple reaction monitoring mode.
  
  
  RESULTS
  The lower limit of quantification was 0.6 pmol/L, and the upper limit of linearity was 600 pmol/L. Total imprecision was <10%. Very poor agreement was observed with the RIA (n = 207; Deming regression RIA = 0.059 × LC-MS/MS - 1.8, r = 0.483; Sy|x = 3.9). Evaluation of the clinical performance of the assay using samples from patients with and without hypercalcemia (n = 199) resulted in an area under the ROC curve of 0.874. In sets of consecutively analyzed routine samples of patients assessed for hypercalcemia, the PTHrP positivity rate by RIA (n = 1376) was 1.9%, and 26.6% by LC-MS/MS (n = 1705). Concentrations were below the lower limit of quantification in 95.6% of the samples by RIA and 2.0% by LC-MS/MS.
  
  
  CONCLUSIONS
  PTHrP is a normal constituent in circulating blood and its concentrations are substantially underestimated by commercial RIAs, causing false-negative results in samples from patients suspected of hypercalcemia. Our observations suggest a link between increased concentrations of PTHrP in postmenopausal women with low body mass index and increased incidence of osteoporosis.
authors.affiliation: ARUP Institute for Clinical and Experimental Pathology, Salt Lake City, UT; Department of Pathology and kushnmm@aruplab.com.
authors.forename: Mark M
authors.initials: MM
authors.lastname: Kushnir
authors.affiliation: ARUP Institute for Clinical and Experimental Pathology, Salt Lake City, UT; Department of Pathology and.
authors.forename: Alan L
authors.initials: AL
authors.lastname: Rockwood
authors.affiliation: ARUP Institute for Clinical and Experimental Pathology, Salt Lake City, UT; Department of Pathology and.
authors.forename: Frederick G
authors.initials: FG
authors.lastname: Strathmann
authors.affiliation: ARUP Institute for Clinical and Experimental Pathology, Salt Lake City, UT; Department of Pathology and.
authors.forename: Elizabeth L
authors.initials: EL
authors.lastname: Frank
authors.affiliation: ARUP Institute for Clinical and Experimental Pathology, Salt Lake City, UT; Department of Pathology and.
authors.forename: Joely A
authors.initials: JA
authors.lastname: Straseski
authors.affiliation: Department of Pathology and Department of Medicine, University of Utah, Salt Lake City, UT.
authors.forename: A Wayne
authors.initials: AW
authors.lastname: Meikle
mesh_terms: D000328:Adult; D000368:Aged; D002851:Chromatography, High Pressure Liquid; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D008297:Male; D008875:Middle Aged; D044162:Parathyroid Hormone-Related Protein / Q000097:blood*; D053719:Tandem Mass Spectrometry; D055815:Young Adult
chemical_list: D044162:Parathyroid Hormone-Related Protein