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Posted by on 18 November 2011
Dr Shaun Treweek is a Senior Lecturer at the University of Dundee and Assistant Director of the Tayside Clinical Trials Unit, and leading a project to improve the way research information from clinical guidelines is presented to the public. Here he takes a closer look at a trial which suggested that dogs may have the ability to detect signals of cancer, and explains what can be learnt from this about the process of blinding in clinical trials.
A Daily Mail article on the 16 November 2011 reported that dogs can detect cancer. Since the alternative is a biopsy, it’s easy to conclude that this is just one more reason to like Labradors. And as Samuel L. Jackson’s character in Pulp Fiction pointed out; a dog’s got personality. Personality goes a long way.
Daisy, the dog featured in the article, correctly picked a vial of urine from a cancer patient from a selection of samples of healthy individuals five times. But did she pick the right vial because she can sniff cancer or because she picked up a sign from the trainer as to which vial was the one to pick? This is a problem common to many clinical trials: If participants know they are on the new treatment they might be more inclined to say they feel better, or have less pain. If clinicians know a patient is taking a new treatment, they might care for the patient differently to those receiving the comparison treatment.
Blinding, where a participant doesn’t know which treatment he or she is receiving, or double-blinding, where the clinicians don’t know either, help to fix this problem. In Daisy’s sniff test, the trainer knows which vial Daisy should pick - in other words, the trainer was unblinded. Daisy doesn’t know so she is blind. It’s easy to imagine a more effective, double-blind test where the trainer doesn’t know either; the 12 vials could just have been numbered and given to him. In this case, if Daisy picked vial 6 each time we might begin to think that vial 6 was from a cancer patient, but we wouldn’t know this until we were ‘unblinded’ after the tests were complete and once the opportunity to influence the result has past. Crucially in a trial, that gives us more confidence in the result.
In the double blinded sniff test, Daisy misses out on treats because noone knows whether she’s right or wrong until the end. Maybe she’d settle for one really big biscuit?