Unmasking (detection signal) bias

An innocent exposure that, rather than causing a disease, causes a sign or symptom that precipitates a search for the disease.


Some exposures cause some groups of people to be given a diagnosis earlier, or at an overall higher rate than other groups of people, and these might not be causes of the disease; if this happens, unmasking, or detection signal, bias has occurred.


If a medication can cause vaginal bleeding, and people with this symptom go sooner to the doctor and receive earlier or more intensive examination and investigations to diagnose cancer, it may appear that the medication caused the cancer. However, all that may have happened is that the medication has prompted an earlier or more intensive search for the disease, leading to an apparently increased rate among those using the medication. This is unmasking (detection signal) bias.


We are unaware of any studies formally investigating the impact of this bias.

Preventive steps

During the design of a study, try to identify factors that might cause signs or symptoms that are also associated with the disease of interest. During analysis of the data keep in mind any such factors, and carry out a sensitivity analysis if indicated.


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