by David G. Rhoads, Ph.D., DABCC, David G. Rhoads Associates, Inc. and Ellie Cheever, MT(ASCP), Dade Behring, Inc.

A task that is often performed in Hemostasis labs is definition of the reference interval for APTT based on a linear regression evaluation relative to heparin concentrations. It is similar in many ways to a method comparison evaluation but is different in several significant ways. This can be done easily using EP Evaluator Release 5 as follows:

Statistical Module: Alternate Method Comparison

1. X Method: Heparin
2. Y Method: APTT
3. Analyte: Instrument identifier (i.e. CA1500)

Set most of the parameters to the usual expected values. The special settings are shown below:

1. Set X Method MDP’s to: 0.3 and 0.7 (This assumes that the heparin concentrations for the lower and upper medical decision points are 0.3 and 0.7. If other values are used, use them instead.)
2. Put a check in the checkbox labeled Disparate Scales
3. Set Rep SD for the Y Method (APTT) to: 50

On your report, the new reference interval will show up as the Y Method MDPs.

Notes:
The new feature in EE5 which enables this calculation is the Disparate Scales checkbox. This has two effects on the process: (1) It forces the medical decision points to be calculated from the Deming regression line without regard to the correlation coefficient (R). Normally with an R less than 0.9, MDPs are calculated using the partitioned bias approach. (2) The range of the X and Y scales are calculated flexibly.