Positive Test (2 of 2) (image) Harvard University Caption This is a capillary tube showing a positive test for sickle cell disease. The system works by using polymers which self-assemble into layers which can separate red blood cells according to their density. The more dense cells infected with sickle cell disease fall to the bottom of the tube. Credit AJ Kumar Usage Restrictions None Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.