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Replicate blind AHG tests with weak IgG anti-D sensitised red cells revealed that 32% of workers caused 5 per cent or more false negative errors by using excessive agitation in reading techniques. The common quality control procedure of adding strongly sensitised cells to all negative AHG tests cannot reveal this type of error. Furthermore, strongly sensitised control cells may create an illusion of safety because AHG giving a false negative test with weak antibody in a serum sample may still show a reassuringly strong positive in the control test. "In-house" assessment of all staff and automatic cell-washers by blind replicate tests is recommended as an effective way of improving AHG test performance, thus reducing many of the errors involving false negative AHG tests seen year after year in External Proficiency Trials.