title: Alternative calibration strategies for the clinical laboratory: application to nortriptyline therapeutic drug monitoring.
abstract: BACKGROUND
  The addition of a calibration curve with every run is both time-consuming and expensive for clinical mass spectrometry assays. We present alternative calibration strategies that eliminate the need for a calibration curve except as required by laboratory regulations.
  
  
  METHODS
  We measured serum nortriptyline concentrations prospectively in 68 patients on 16 days over a 2-month period using a method employing calibration schemes that relied on the measurement of the response ratio (RR) corrected by the response factor (RF), i.e., a measurement of the RR for an equimolar mixture of the analyte and internal standard. Measurements were taken with contemporaneous RF (cRF) measurements as well as sporadic RF (sRF) measurements. The measurements with these alternative calibration schemes were compared against the clinical results obtained by interpolation on a calibration curve, and those differences were evaluated for analytical and clinical significance.
  
  
  RESULTS
  The differences between the values measured by cRF and sRF calibration and interpolation on a calibration curve were not significant (P = 0.088 and P = 0.091, respectively). Both the cRF- and sRF-based calibration results demonstrated a low mean bias against the calibration curve interpolations of 3.69% (95% CI, -15.8% to 23.2%) and 3.11% (95% CI, -16.4% to 22.6%), respectively. When these results were classified as subtherapeutic, therapeutic, or supratherapeutic, there was categorical agreement in 95.6% of the cRF calibration results and 94.1% of the sRF results.
  
  
  CONCLUSIONS
  cRF and sRF calibration in a clinically validated liquid chromatography-tandem mass spectrometry assay yields results that are analytically and clinically commensurate to those produced by interpolation with a calibration curve.
authors: Department of Pathology, the Johns Hopkins Hospital, Baltimore, MD, USA.
authors: Matthew T
authors: MT
authors: Olson
authors: Autumn
authors: A
authors: Breaud
authors: Robert
authors: R
authors: Harlan
authors: Nkechinyere
authors: N
authors: Emezienna
authors: Sabitha
authors: S
authors: Schools
authors: Alfred L
authors: AL
authors: Yergey
authors: William
authors: W
authors: Clarke
mesh_terms: D019963:Clinical Chemistry Tests / Q000191:economics / Q000592:standards* / Q000706:statistics & numerical data; D016903:Drug Monitoring / Q000191:economics / Q000592:standards*; D006801:Humans; D013058:Mass Spectrometry / Q000191:economics; D009661:Nortriptyline / Q000097:blood*
chemical_list: D009661:Nortriptyline