Each FISH assay must use a positive control slide. After completing a FISH assay, the priority is to assess the quality of the control slide.
The positive control slide may be of several types. A cell line control may be used. This control slide may be manufactured at the testing site if prepared into a pellet and formalin-fixed, paraffin-embedded (FFPE) in the same manner as the sample tissues. Some manufacturers offer a positive control slide that can be purchased in addition to the probe set. These are most often cell lines as well. A tissue control that contains abnormalities can be used as a positive control. This is an optimal situation because the tissue resembles the test samples. However, they can be hard to find and keep enough on hand for long-term purposes.
The laboratory performing the FISH assay must determine the control slide enumeration range. Typically, laboratories use a control cell line with a 4.0 +/–1 standard deviation (SD) range. If the cell line control is purchased from a manufacturer, the manufacturer will provide a control range. Still, the laboratory must verify the control range and determine its in-house standard deviation.
If the control slide enumeration does not come into the pre-determined quality control range, the entire assay must be rejected and repeated.
