Answer:
In the case that the samples show an unusual behavior
Step-by-step explanation:
For example when there is a blood transfusion, maybe the first filter the patient pass the probes, but when the blood goes to the lab for a deep analysis to have it as a reservoir, then another situation appears like AIDS, a bacteria, another virus, some morphological deformities in blood cells, etc. In this cases is ethical to re - contact the patients, in order to alert of the results and go for a second check.