title: Reference change values and power functions.
abstract: Repeated samplings and measurements in the monitoring of patients to look for changes are common clinical problems. The "reference change value", calculated as zp x [2 x (CVI2 + CVA2)](1/2), where zp is the z-statistic and CVI and CVA are within-subject and analytical coefficients of variation, respectively, has been used to detect whether a measured difference between measurements is statistically significant. However, a reference change value only detects the probability of false-positives (type I error), and for this reason, a model to calculate the risk of missing significant changes in serial results from individuals (probability of false-negatives) is investigated in this work by means of power functions. Therefore, when an analyte is being monitored in a patient, power functions estimate the probability of detecting a defined real change by measuring the difference. Thus, when a measured difference is the same as the calculated reference change value, then it will be detected in only 50% of situations.
authors.affiliation: Laboratoris Clínics, Hospital General Universitari Vall d'Hebron, Barcelona, Spain. iglesias@hg.vhebron.es
authors.forename: Natàlia
authors.initials: N
authors.lastname: Iglesias Canadell
authors.forename: Per
authors.initials: P
authors.lastname: Hyltoft Petersen
authors.forename: Esther
authors.initials: E
authors.lastname: Jensen
authors.forename: Carmen
authors.initials: C
authors.lastname: Ricós
authors.forename: Per E
authors.initials: PE
authors.lastname: Jørgensen
mesh_terms: D000704:Analysis of Variance; D002118:Calcium / Q000097:blood*; D003404:Creatinine / Q000097:blood*; D003627:Data Interpretation, Statistical; D005189:False Positive Reactions; D006441:Hemoglobin A / Q000032:analysis*; D006801:Humans; D008954:Models, Biological; D011237:Predictive Value of Tests; D011336:Probability; D012016:Reference Values
chemical_list: D006441:Hemoglobin A; D003404:Creatinine; D002118:Calcium