Fluoride Test (image) Northwestern University Share Print E-Mail Caption The test tube on the left shows a real positive result from water sampled in Costa Rica. The middle tube is a negative control. The tube on the right is a positive control. Credit Julius B. Lucks/Northwestern University Usage Restrictions None Share Print E-Mail 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.