We get a lot of questions about control tissue on The Block, NSH’s member community, and our Facebook Professionals Group. In this blog post we will answer some of these commonly asked questions.
What is the best type of control tissue for H&E?
The majority of our Facebook community said appendix, with others using both appendix and colon. Skin would be appropriate for a derm lab, but most routine histology labs use appendix.
What documentation is needed for verification of control slides?
According to CAP, positive control slides for special stains and IHC must be verified prior to being used for patient diagnosis. A positive control verification form is filled out prior to use which contains information such as the stain name, source, lot number, etc. The director must sign off on the form before the slide can be used.
Should control slides be refrigerated?
Some controls such as placenta, p53, and p57, for example, may lose antigenicity (the antigen’s ability to bind with the products of the cell mediated response, like B-cells and T-cells), if they are left out. Others are fine being left on the shelf. Whether or not you need to refrigerate them also depends on how far in advance they have been cut. Ultimately, the best way to determine your lab’s needs is to make observations about the staining quality and alter your processes accordingly. Cut your IHC controls and store some at room temperature and others in the freezer for the length of time you want to test (however long you envision having to store them). At regular intervals within that period, perform IHC with critical markers and observe the results.
If you’re going to be putting them in a fridge or freezer, let them air dry before putting them in.
Where do you get AFB and fungus controls?
If you have access to a microbiology lab, you can give them some tissue and let them grow a fungus control for you. Strawberries and orange peels are also common sources of fungus. Moisten an orange peel and leave it out for a week to mold, then fix, process, embed and cut for positive control. Similarly, with strawberries, cut off the mold, wrap it in tissue like appendix and process as usual.
AFB can be made from positive autopsy tissue. Alternatively, if you can get in contact with a veterinary pathologist, small birds and fish can get mycobacterium which can be used. If none of these are an option for you there are also companies that sell this type of control.
Do you need to run a negative control with your special stains?
This question came up in relation to deficiencies during CAP inspection related to ANP.21395, which says “For special stains, including histochemical stains, and studies using immunohistologic and ISH methodology, positive and negative controls are verified and recorded as acceptable prior to or concurrent with the reporting of patient results and records retained.”. Most labs do not run negative controls for special stains except for microorganism stains like Gram, AFB and Fite that have different organisms that stain differently positive and negative. The argument against the need for negative controls for special stains is that there are internal negatives within the positive control where there is no organism activity. If you are going to rely on this method, make sure this is documented in your procedures, particularly if you are going to be CAP inspected.
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