Final answer:
When no agglutination is observed after adding Coomb's control cells to a negative Ab screen, the testing procedure should be re-evaluated to confirm proper technique and active reagents, particularly verifying that Coombs' reagent was correctly used and is functional.
Step-by-step explanation:
If no agglutination is observed after adding Coomb's control cells to a negative antibody screen, this suggests that the test conditions are inadequate for the detection of sensitized red blood cells. In such a case, the course of action would typically be to re-evaluate the testing procedure to ensure proper technique and reagents were used. Specifically, attention should be paid to the addition of Coombs' reagent which contains antihuman antibodies that should facilitate the visualization of antibody-red blood cell interactions.
It is crucial to confirm that the Coomb's control cells, which are sensitized red blood cells, react positively with the Coombs' reagent to validate the test's effectiveness. If the control cells do not agglutinate as expected with the Coombs' reagent, the test is considered invalid, and one must first troubleshoot and resolve any technical issues before interpreting patient results. This may involve checking the reagent's activity and ensuring that the technique aligns with the standard procedure.