Which coliform test methods have three distinct steps: presumptive, confirmed, and completed?

Which coliform test methods have three distinct steps: presumptive, confirmed, and completed?

• A. MF
• B. MNO-MUG
• C. MTF
• D. MF and MTF

Answer: (D)

The test is looking for methods that use a staged verification process—presumptive, confirmed, and completed—to identify coliforms. In the membrane filtration family, these steps are built into the workflow. With the membrane filtration (MF) method, you filter a water sample and place the filter onto a presumptive medium to screen for lactose fermentation, which signals potential coliforms. If the presumptive result is positive, you then transfer or submerge the sample on a confirmatory medium that’s more selective for coliforms to verify their presence. Finally, the completed step provides a final verification, often involving subculturing to another medium or performing an additional test (such as a MUG-based assessment for E. coli) to confirm the isolate’s identity. The sequence—presumptive screening, confirmatory testing, and final confirmation—is why MF employs three distinct stages. The membrane filter test (MTF) follows the same three-step structure: initial presumptive screening after filtration, a confirmatory step to verify coliforms, and a completed step to finalize identification and isolation. This consistent three-stage design is what distinguishes these two methods. Other methods, such as MNO-MUG, are typically used in different testing frameworks (often MPN-based and focused on rapid or single-platform detection) and aren’t described as a three-step presumptive-confirmed-completed process in the same way. That’s why MF and MTF are identified as having three distinct steps.

The test is looking for methods that use a staged verification process—presumptive, confirmed, and completed—to identify coliforms. In the membrane filtration family, these steps are built into the workflow.

With the membrane filtration (MF) method, you filter a water sample and place the filter onto a presumptive medium to screen for lactose fermentation, which signals potential coliforms. If the presumptive result is positive, you then transfer or submerge the sample on a confirmatory medium that’s more selective for coliforms to verify their presence. Finally, the completed step provides a final verification, often involving subculturing to another medium or performing an additional test (such as a MUG-based assessment for E. coli) to confirm the isolate’s identity. The sequence—presumptive screening, confirmatory testing, and final confirmation—is why MF employs three distinct stages.

The membrane filter test (MTF) follows the same three-step structure: initial presumptive screening after filtration, a confirmatory step to verify coliforms, and a completed step to finalize identification and isolation. This consistent three-stage design is what distinguishes these two methods.

Other methods, such as MNO-MUG, are typically used in different testing frameworks (often MPN-based and focused on rapid or single-platform detection) and aren’t described as a three-step presumptive-confirmed-completed process in the same way. That’s why MF and MTF are identified as having three distinct steps.