The accuracy of Colilert-18 as a test for coliforms and Escherichia coli in subtropical freshwater was evaluated by using API 20E strips and fatty acid methyl ester analysis. The false-positive and -negative rates of detection were 7.4 and 3.5%, respectively, for E. coli and 9.6 and 6.3%, respectively, for coliforms.In Taiwan, coliform bacteria are the recommended hygienic indicator organisms for both raw and potable water. However, many bacteria in the genera of Klebsiella, Enterobacter, Citrobacter, and Serratia meet the definitional criteria for coliforms even though they are not of fecal origin (8). Disagreements between government regulators and the water company sometimes relate to water that contains nonfecal coliforms, which interfere with the ability to accurately detect coliforms of fecal origin (the permissible limit in Taiwan being 20,000 CFU per 100 ml of raw water). As suggested previously, Escherichia coli could serve as a more specific indicator for fecal contamination of freshwater. The Republic of China Environmental Protection Agency has approved the use of two methods for the detection and enumeration of E. coli in water samples. One of them is a most-probable-number method based on the defined substrate technology using o-nitrophenyl-␤-D-galactopyranoside (ONPG) and 4-methylumbelliferyl-␤-glucuronide (MUG) as the growth substrates (7).A proprietary medium formulation and testing package incorporating these substrates has been marketed by IDEXX Laboratories (Westbrook, Maine) under the product name Colilert. The Colilert system yields results that are consistently similar to those of traditional standard detection methods for E. coli and total coliforms in freshwater (4, 5, 6). Colilert-18, a system intended to provide rapid (18-h) results, has been reported to detect coliforms and E. coli in potable water samples with an accuracy of 100% (10). However, other studies have disputed this degree of accuracy (14,16). This dispute was of concern in our efforts to achieve rapid and accurate E. coli and coliform detection in our water monitoring program in Taiwan. Herein, we report the false-positive and false-negative rates of E. coli and coliform detection in subtropical freshwater samples obtained near metropolitan Taipei using Colilert-18.Isolation of target organisms. Thirteen river water samples were collected from 11 different sites between June 2002 and January 2003. At each sampling date, following at least 3 consecutive days of clear weather, water samples were collected between 9 and 11 a.m. Water temperatures in the region averaged 28.5 Ϯ 1.8°C (July-September) and 22.3 Ϯ 0.9°C (October-December). Each sample was collected by immersing a 500-ml autoclaved polypropylene bottle in the middle of the river. The samples were placed on ice and transported back to the laboratory. The entire process, from sampling to the commencement of incubation, took less than 6 h. Detection of coliforms and E. coli by using the Colilert-18 (Quanti-Tray/ 2000) method was done according to the manufacturer's i...