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  Queries and Comments

 

June 2003
Originally published in CAP TODAY

From the files of the CAP’s checklist-related questions. Answers by Stephen J. Sarewitz, MD, LAP checklist commissioner and staff pathologist, Valley Medical Center, Renton, Wash.

Q: Please expand on the following checklist question (CHM. 24400; Phase II): If automatic pipetting is used, has the laboratory evaluated the testing system for carryover effects?

A: The laboratory must have a procedure in place for evaluating whether carryover effects are present. One suggested method is to run known high samples— calibrators, standards, reference material, assayed controls—followed by known low samples to see if the results of the low-level material are affected. If carryover is detected, the laboratory must determine the level past which low-level samples are affected and this must be defined in the procedure. Results of each analytical run must be reviewed to ensure that no results exceed this level. If results that exceed the defined level are detected, then the appropriate course of action, such as repeating subsequent samples, must be defined.

Q: Must carryover studies be performed for all analytes? How often should these studies be performed?

A: In drug testing, all drugs ideally would be subject to carryover testing, but this may not be practical—for example, if the laboratory is performing general toxicology screening. In such cases, representative drugs may be studied. A laboratory may choose to run blanks after positive specimens to avert problems related to carryover.

For other types of tests, representative analytes may be studied. The analytes selected, such as human chorionic gonadotropin, should have a wide range of concentrations. The laboratory director is responsible for determining which analytes should be studied.

Carryover studies must be performed at the time the instrument is initially evaluated. Repeat carryover studies may be performed at the discretion of the laboratory director. Because instrument performance may deteriorate with age, carryover studies probably should be repeated periodically. The CAP Standards/Instrumentation Committee is developing a carryover challenge.

Q: Is fluorochrome staining specifically required for direct mycobacterial smears (microbiology checklist question MIC.32100)? Is it acceptable to stain unconcentrated sputum specimens?

A: Fluorochrome stains are specifically required on smears prepared from primary specimens. Fluorochrome stains are more sensitive than conventional carbol-fuchsin-based stains. The stain may be performed in-house or at a reference lab. Sputum specimens should be concentrated before smears are prepared.

To improve clarity, this checklist question will be reworded in the next edition of the microbiology checklist, which will be released later this year. The question (MIC.32100; Phase II) will read: "Is fluorochrome staining performed on mycobacterial smears prepared from primary specimens, either in the laboratory or by the reference laboratory?"