AHG tests may be controlled routinely by the use of 3 controls:
Addition of IgG sensitized cells
Known IgG sensitized calls are added to all AHG tests that are negative both macroscopically and microscopically. The test is then centrifuged and read macroscopically for agglutination, which must occur. These cells control two facets of the test: a) they control that AHG serum was added to the test, and b) they control that washing was adequate and no unbound protein was left behind which would neutralize the AHG serum. If no agglutination results, the AHG test is invalid and must be totally repeated from the start. (See Figure 3-2.)
Positive Control
This ensures the reactivity of the anti-IgG AHG serum. A weak IgG anti-D (1/250) is incubated with D positive cells (OR'r) and tested in parallel with the other AHG tests. After the addition of AHG serum, the test must be positive.
Negative Control
This ensures the specificity of the AHG serum, i.e., that it does not contain unwanted antibodies. The same 1/250 anti-D used for the positive control is incubated with D-negative cells (Arr) and tested in parallel with the other AHG tests. After addition of the AHG serum, the test must be negative. NOTE: If negative macroscopically and microscopically (expected result), IgG sensitized cells are also added to the negative control.
Note: Many labs do not include positive and negative controls in parallel with indirect antiglobulin tests. Instead, the reactivity and specificity of antiglobulin sera may be tested as part of daily quality control or whenever a new lot number is opened. In such cases, a variety of IgG antibodies may be used to control the reagent's reactivity. Policies on routine QC of AHG testing vary greatly.
Adding IgG sensitized red cells to negative antiglobulin tests is essential QC when tests are done in tubes. However, for tests done in microtubes (e.g., gel and other column technology), adding IgG sensitized red cells is not part of the test procedure.
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| Quality Control of the IAT |