Commutability of proficiency testing material containing tobramycin: a study within the framework of the Dutch Calibration 2.000 project

Background:Results from external quality assessment schemes (EQASs) can provide information about accuracy and comparability of different measurement methods, provided that the material used in these schemes behave identical to patient samples among the different methods, a characteristic also known as commutability. The aim of this study was to assess the commutability of different matrices for the material used in an EQAS for tobramycin.Methods:Proficiency testing material (PTM) and patient samples containing tobramycin were prepared, collected, pooled, and distributed to participating laboratories for analysis. Low, medium, and high tobramycin concentrations in liquid human, liquid bovine and lyophilized bovine serum were tested in this study. The patient serum results of every laboratory were plotted against each of the other laboratories, and the distances of the PTM results to the patient serum regression line were calculated. For comparison, these distances were divided by the average within-laboratory standard deviation (SDResults:With 10 laboratories participating in this study, 45 laboratory couples were formed. For human serum, only one relative residual for high concentrations of tobramycin was found outside the commutability decision limit. For liquid and lyophilized bovine sera, the number of relative residuals outside the decision limit was between 15 and 18 for low, medium, and high tobramycin concentrations.Conclusions:The PTM used for tobramycin is preferably prepared with human serum.

Reducing the Variation in Performance of Antibody Titrations

Background.—Antibody titration is difficult to standardize. We investigated whether a detailed, uniform procedure for antibody titration would reduce variation in both tube-based and gel card titres in an international study. Methods.—Laboratories (n = 35) tested proficiency testing material provided by the College of American Pathologists each according to (i) their routine method; (ii) a detailed, uniform method; and (iii) the uniform method titrating the serum sample against a red cell of specified phenotype (D+ C− c+ E+ e− for anti-D; A1 for anti-A) instead of the red cell of the same phenotype provided in the proficiency testing kit. Uniform method results were reported with 1+ and w+ end-points. Paired statistical analyses of variance were conducted using the F-test. Results.—The variance between laboratories was not significantly reduced with the uniform method using a 1+ end-point. However, a statistically significant reduction in the variance of anti-D and anti-A titres by the tube-based uniform technique after 37°C incubation and conversion to the antiglobulin (AHG) phase was seen when 19 laboratories reanalysed their results using a w+ end-point. Too few laboratories reported results with a w+ end-point in gel card testing to allow analysis. Titration against red cells of the specified phenotype provided by the participating laboratory did not appear to introduce additional variance. Overall, results reported based on the gel card technique at the AHG phase (1+ end-point) showed reduced variance compared to tube-based techniques. Conclusions.—A detailed, uniform method for antibody titration at 37°C and read at the AHG phase in a tube-based method with a w+ end-point reduced interlaboratory variability.

Point-of-care International Normalised Ratios: UK NEQAS experience demonstrates necessity for proficiency testing of three different monitors

SummaryExternal quality assessment (EQA) or proficiency testing is widely considered to be necessary for International Normalised Ratio (INR) determinations performed in conventional laboratory settings. There is increasing use of near-patient-test (NPT) or point-of-care (POC) INR devices and it is not known whether EQA is also necessary for these monitors. We report here on six years experience of proficiency testing for POC monitors used by health care professionals. Three devices were used by >10 centres who participated in the programme, the CoaguChek (CUC), the CUC-S and the TAS or Rapidpoint Coag. Not all users of the same type of monitor obtained the same INR result when analysing the same plasma sample. For the three monitors the CV of results in different centres was 11–14%. The variation between results in different centres could relate to inappropriately handled proficiency testing material, inaccuracies in the calibration of the system by the manufacturer or deterioration during transport/storage of the test strips. In each survey 10–11% of centres using POC monitors obtained INR results which were >15% different from those in other centres using the same monitors. For hospital laboratories using conventional INR techniques this figure was 12%. The relationship between INR results obtained by users of the Rapidpoint Coag orTAS monitor and results obtained by conventional techniques was not constant over the period of study. During one period INRs with TAS were 13.7% greater than with conventional methods. For the remaining three time periods results were similar. Our data suggest that the variation between INR results determined with three POC monitors show similar variation to that observed in hospital laboratories using conventional methods. Based on our data we recommend that users of these POC monitors participate regularly in an independent external proficiency testing programme.

Comparison of Fresh Frozen Serum to Traditional Proficiency Testing Material in a College of American Pathologists Survey for Ferritin, Folate, and Vitamin B12

Context.—Comparison of different analytical methods in proficiency surveys may be affected by the artificial nature of the survey material. Objective.—To compare intermethod differences in proficiency survey results between 2 types of survey material, conventional proficiency testing material (PTM) and fresh frozen human serum (FFS), for 3 markers of anemia: ferritin, folate, and vitamin B12. Design.—Data were gathered from a 2003 survey event in the College of American Pathologists Ligand (“K”) Series, in which the specimens to be tested by each participating laboratory included 1 vial of FFS and 2 vials of PTM with different analyte concentrations. The more than 1600 laboratories subscribing to the survey were not advised as to the nature of the specimens. Main Outcome Measures.—The bias of each method relative to the median of method means for each analyte and each type of survey material, and the interlaboratory coefficient of variation for each method. Results.—For each of the 3 analytes, moderate to large method biases were observed. For ferritin, method biases correlated strongly between comparable PTM and FFS specimens (Spearman r = 0.863, P < .001), whereas virtually no correlation was found for folate (r = −0.224, P = .48), and a marginally significant correlation existed for B12 (r = 0.55, P = .049). Conclusions.—With ferritin, proficiency survey performance of PTM is similar to that of FFS, implying that method biases relate mainly to calibration. With folate and to a lesser extent with B12, PTM and FFS exhibit different method biases, implying that the biases reflect analyte heterogeneity and/or matrix effects.

A Comparison of Human Chorionic Gonadotropin– Related Components in Fresh Frozen Serum With the Proficiency Testing Material Used by the College of American Pathologists

Context.—As part of a College of American Pathologists (CAP) proficiency testing survey, a comparison is made between human chorionic gonadotropin (hCG) results from an actual patient pool and a similarly targeted artificial sample. The goal is to gain insight into the possible source(s) of bias attributable to the proficiency testing material (PTM) with a view toward creating more appropriate survey materials. Objective.—To compare hCG and related variants in a pool of fresh frozen sera (FFS) with that found in PTM. Design.—The 2003 CAP K/KN-A Survey included a FFS specimen along with admixtures of PTM. The FFS (K-02) and 1 PTM admixture (K-01) had similar mean hCG values. Five hCG-related analytes were measured on these 2 samples by a reference laboratory. Participants.—Approximately 1800 clinical laboratories and diagnostic test kit manufacturers participated in the K/ KN-A Survey. Main Outcome Measures.—Method imprecision (coefficient of variation) and method bias (relative difference between peer group mean and all-method median) were computed for the 2 samples. Differences were evaluated with respect to hCG-related analytes levels. Results.—All-method hCG results were 12.9 mIU/mL (12.9 IU/L) for the PTM material and 21.6 mIU/mL (21.6 IU/L) for the FFS material. Method biases for 14 manufacturers were greater for PTM than for FFS (−40% to +35% and −16% to +23%). Twelve of 14 methods had higher coefficients of variation on PTM. Total hCG and free β hCG measurements by the reference laboratory were 14.1 mIU/ mL (14.1 IU/L) for the PTM material and 18.5 mIU/mL (18.5 U/L) for the FFS material (FFS), and 2.4 (PTM) and 0.7 (FFS) mIU/mL (2.4 and 0.7 IU/L), respectively. On a molar basis, free β represented 17% and 4% of the total hCG, respectively. Levels of hyperglycosylated hCG, nicked hCG, and β core fragment were not measurable in either sample. Conclusions.—It is unlikely that the hCG added to the PTM is the source of the increased bias and variability. The main difference is a 3-fold increase in free β found in the PTM, but methods previously found to strongly react with free β were not systematically elevated. The biases between manufacturers found for the FFS specimen are likely attributable to calibration differences.