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HER2 neu IHC stained tissue section.

Use of Immunohistochemistry (IHC) for Determining Estrogen Receptor (ER) and Progesterone Receptor (PR) Status

IHC is the most commonly used method for measuring and detecting ER and PR status in breast tumors. In the 1970s, the "positive” and “negative” cut off points for ER and PR were established by ligand binding assays (LBAs). By the mid-1990s, LBAs were replaced by IHC, as breast cancer studies began to consistently show IHC to be equivalent to superior to LBAs in predicting response to hormone therapy (with levels as low as 1% of positive staining cells).
IHC has the advantage of being relatively rapid and a cost effective method of testing. And unlike the older LBA biochemical assays, IHC can be carried out on small tumors, including breast needle core biopsies.
Some drawbacks to IHC methods include an element of subjectivity in scoring (it is a qualitative test method) and variability in results, especially in comparisons between laboratories, due to variations in fixation and tissue processing.
Source: Clinical and Laboratory Standards Institute (CLSI). Quality Assurance for Design Control and Implementation of Immunohistochemistry Assays. 2nd ed. CLSI document I/LA28-A2. Wayne, PA: CLSI: 2011.