1: Clin Chem 1997 Nov;43(11):2175-82 Related Articles, Books, LinkOut: Biological variation of International Normalized Ratio for prothrombin times, and consequences in monitoring oral anticoagulant therapy: computer simulation of serial measurements with goal-setting for analytical quality.

Kjeldsen J, Lassen JF, Petersen PH, Brandslund I.

Department of Medical Gastroenterology S, Odense University Hospital, Denmark.

Oral anticoagulant therapy (OAT) has a well-established efficacy in prophylaxis and treatment of thromboembolic disorders. Because complications are related to intensity of OAT, optimal control of treatment is mandatory. In studies of OAT, as many as 30% of International Normalized Ratio (INR) measurements for prothrombin times fall outside the therapeutic interval. Preanalytical, analytical, and biological variation all contribute to this. Computer simulations of serial INR measurements were performed for various assumed in-treatment setpoints within the therapeutic interval INR 2.0-3.0 and for an "in-treatment within-subject variation" (CV) of 10.1%. Results are presented in difference plots with therapeutic intervals and critical differences. If the in-treatment setpoint is mid-interval (INR = 2.5), only 5% of simulated INR values fall outside the therapeutic interval. Setpoints deviating from the mid-interval and increases in the in-treatment within-subject variation considerably increase the number of observations outside the therapeutic interval and the critical differences. In conclusion, random variation, biological or analytical, and setpoints (targets) deviating from mid-interval explain a substantial number of the INR values outside therapeutic intervals observed in clinical studies. Analytical imprecision should be kept < 5% and analytical bias < +/- 0.2 INR.

PMID: 9365405 [PubMed - indexed for MEDLINE]

2: Clin Chem 1997 Nov;43(11):2064-75 Related Articles, Books, LinkOut

Comment in:

Clin Chem. 1997 Nov;43(11):2017-8.

Evaluation of a near-patient test for C-reactive protein used in daily routine in primary healthcare by use of difference plots.

Dahler-Eriksen BS, Lassen JF, Petersen PH, Lund ED, Lauritzen T, Brandslund I.

Department of Clinical Chemistry, Vejle County Central Hospital, Denmark.

We have assessed the technical performance and robustness of NycoCard CRP Whole Blood, a near-patient test for C-reactive protein (CRP), when used in realistic daily routine situations in general practice clinics (GPC). Thirteen GPCs participated, five of them with technician staff. From 898 patients, split-sample measurements for CRP were made. Results from GPCs were compared with results from a turbidimetric laboratory method, traceable to international reference preparations (IFCC CRM 470). Results were evaluated in difference plots where the expected distribution, due to an estimated analytical variation, was compared with measured differences. Of all difference points, 91.5% (n = 819) were within a 95% prediction interval based on the imprecision of both methods. Mean bias (95% confidence interval) was -0.3 mg/L (-0.9 to 0.3). No differences in analytic quality were found between GPCs with technician staffs and GPCs without, and between test results obtained within the first and second week, compared with the rest of the study period. We find the test as good when used in GPCs as could be expected from laboratory testing, and consequently robust, which is a necessity for use in routine situations in general practice. General application of difference plots in test evaluations are discussed in detail.

PMID: 9365390 [PubMed - indexed for MEDLINE]

3: Clin Chem 1997 Nov;43(11):2039-46 Related Articles, Books, LinkOut: Graphical interpretation of analytical data from comparison of a field method with reference method by use of difference plots.

Petersen PH, Stockl D, Blaabjerg O, Pedersen B, Birkemose E, Thienpont L, Lassen JF, Kjeldsen J.

Department of Clinical Chemistry, Odense University Hospital, Denmark. [email protected]

Various viewpoints have been offered regarding the appropriate use of scatter plots or difference plots (bias plots or residual plots) in comparing analytical methods. In many of these discussions it seems the basic concepts of identity (within inherent imprecision) and acceptability based on analytical goals (analytical quality specifications) have been forgotten. With the increasing number of Reference Methods in laboratory medicine, these basic concepts are becoming more important in validation of field methods. Here we describe a simple and effective graphical test of these hypotheses (identity and acceptability) by use of difference plots. These plots display the underlying hypothesis before the measured differences are plotted and allow interpretation of the results according to specific criteria. We further describe simple but effective interpretations of the data, when the hypothesis is not fulfilled, by using two data sets drawn from comparisons of field methods for S-creatinine with a Reference Method for this analyte. The difference plot is a graphical tool with related simple statistics for comparison of a field method with a Reference Method, focusing on (a) identity within the inherent analytical imprecision or (b) acceptability within analytical quality specifications. Calculation of the standard deviation of the differences is an indispensable tool for evaluation of aberrant-sample bias (matrix effects).

PMID: 9365386 [PubMed - indexed for MEDLINE]

4: J Immunol Methods 1995 Jan 27;178(2):211-8 Related Articles, Books, LinkOut: Difference and ratio plots: simple tools for improved presentation and interpretation of scientific data. Unexpected possibilities for the use of the conglutinin binding assay in inflammatory rheumatic diseases.

Svendsen A, Holmskov U, Petersen PH, Jensenius JC.

Department of Medical Rheumatology, University Hospital, Odense, Denmark.

11111onglutinin-binding assay (KgBa) has gained widespread use for the detection of circulating immune complexes. A recent paper questioned the interpretation of the results obtained by this method and the validity of the assay (Holmskov et al. (1992) J. Immunol. Methods 148, 225). We now present hitherto unnoted differences between controls and patients with either rheumatoid arthritis or systemic lupus erythematosus. For this we use simple, but unconventional, graphic representations of the data, based on difference plots and ratio plots. Differences between patients with Burkitt's lymphoma and systemic lupus erythematosus from another previously published study (Macanovic, M. and Lachmann, P.J. (1979) Clin. Exp. Immunol. 38, 274) are also represented using ratio plots. Our observations indicate that analysis by regression analysis may often be misleading.

PMID: 7836783 [PubMed - indexed for MEDLINE]

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