Sample interaction (carryover) on the aca when glycol-stabilized serum is used.

Recently we became aware that serum triglyceride results are falsely increased when determined immediately following any of the ethylene glycol-treated liquid-control products (Beckman Instruments, Inc., Brea, CA 92621) in the aca (Du Pont Co., Clinical Systems Division, Wilmington, DE 19898). We believe this is the first report of a significant sample-interaction effect for the aca, and that others using the glycolstabilized serum products on this particular discrete analyzer should be aware of the potential for erroneous results when triglycerides are being assayed. In the aca, serum triglycerides are measured by a kinetic glycerol dehydrogenase method, which measures the total glycerol in the pack after hydrolysis of glyceryl esters by added lipases. Du Pont literature notes that glycerol and other polyols will react “... and may cause positive interference” (1). Instructions with the stabilized liquid control serum warns that “Due to the presence of ethylene glycol, triglyceride determinations involving the use of glycerol dehydrogenase may not be used with these control products” (2). To these warnings we would add that the use of glycol-treated serum may affect triglyceride determinations on the next serum sample placed on the aca. The extent of sample interaction is illustrated by the triglyceride results obtained on the same serum sample run immediately before and after a single creatine kinase assay on Beckman custom unassayed control serum for comprehensive chemistry. The average increase in a triglyceride result for a sample run immediately after a glycoltreated control serum was 160 mgIL. If a second triglyceride pack was run (on the same sample cup), the second result averaged 230 mg/L higher than the pre-glycol value for the same serum. We attribute these increases to a minute amount of the viscous ethylene glycol adhering to the outside of the sample needle as it pierces the plastic lid covering the next sample cup. Before the second triglyceride pack is filled, the sample needle is inserted into the same hole previously made in the sample cup lid; and the glycol deposited around this hole during the first needle insertion may again be picked up and carried into the triglyceride pack. Wiping the outside of the needle clean after the glycol-stabilized serum was run eliminated the triglyceride interference completely. To determine how much glycol-stabilized serum would be required to show the same effect when carried forward into the next sample pack, we diluted Beckman liquid control serum and sampled these dilutions for tn 1 ceride analyses. As little as 2 L of the glyco stabilized serum will give the degree of sample interaction we have been seeing with this combination of viscous control material and an ethylene glycol-sensitive triglyceride method. We currently prefer the stabilized liquid control sera for all appropriate tests and instruments in our laboratory, because the precision of results has improved noticeably since their introduction into our laboratory quality-control program. We have chosen to continue using the Beckman control products in the aca, but with warnings posted to remind aca operators that the sample immediately following these controls should not have a triglyceride analysis requested.