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1262) A slide that has been immunostained on a programmable autostainer is received with a positive control on a separate slide. The positive control shows the expected staining; the internal control cells you know to be present in the test tissue are negative. Which of the following could account for this discrepancy?

“Case” tissue may have been overfixed. “Case” tissue may have not been fixed long enough, or have been fixed in a fixative other than formalin. There may have been a “programming” error when setting up the staining run. A reagent may have run out after the staining of the positive control. All of the above.

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• Variability in fixation may account for false negative staining of test tissues with proper staining of assay controls.

• Problems include delayed fixation, over- and under fixation, and the use of fixatives other than that used to fix the positive control.

• Tissues not subject to formalin fixation may require a different—or no—antigen retrieval. This may be an issue with some newer “rapid processing” methods.

• Programmable autostainers, which do not use a bar code or matrix to identify slides, require special care to ensure slides are placed on the proper position on the slide rack.

• Although shortage of a reagent generally causes an error message in autostainers, the message may be missed if the technician is away, and on some instruments the procedure may be taken to completion with one or more reagents not applied to the slide.

Taylor CR, Shi S-R, Barr NJ, Wu N: “Techniques of immunohistochemistry: principles, pitfalls, and standardization.”In: Dabbs DJ (ed): Diagnostic Immunohistochemistry. New York: Churchill Livingstone, 2002, pp 3-44.

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Anatomic Pathology: Immunopathology

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