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D5392-24 – Standard Test Method Technical Guide
📐 Scope and Significance of the Test Method
This test method, designated D5392-24, describes a two-step membrane filter (MF) procedure for the detection and enumeration of Escherichia coli in water. E. coli is a bacterium found exclusively in the feces of humans and other warm-blooded animals. The presence of these microorganisms in a water sample serves as a critical indicator of fecal pollution and suggests the possible presence of enteric pathogens.
The detection limit of this procedure is one colony forming unit (CFU) per volume filtered, making it suitable for a wide range of densities found in water and wastewater. The standard has been validated for use with temperate fresh and marine ambient waters, as well as wastewaters. It is the responsibility of the user to determine the validity of this test method for other water types. All values stated in the standard are expressed in SI units.
⚠️ Safety and Regulatory Responsibility: The standard explicitly notes that it does not purport to address all safety concerns. Users must establish appropriate safety, health, and environmental practices, and determine the applicability of regulatory limitations. Specific hazard statements can be found in Section 9 of the full standard document.
⚙️ Procedure, Terminology, and Referenced Standards
The two-step membrane filter procedure relies on a robust framework of referenced ASTM standards for proper execution. Sampling must be conducted in accordance with Practices D3370, and the terminology aligns with the definitions found in Terminology D1129. The method requires the use of reagent water as defined by Specification D1193. Colony counting guidelines are strictly governed by Practices D5465.
The standard outlines specific performance characteristics established under Practice D3870 (Withdrawn 2000). Critical terms defined for this method include Accuracy (the proportion of the observed count to the true density), Bias (persistent deviation from the true value), Precision (the degree of agreement of repeated measurements), Specificity (the ability to distinguish target bacteria), and the Upper Counting Limit (UCL).
💡 Technical Note on Enumeration: The Upper Counting Limit (UCL) is a critical operational parameter for this test method. It is defined as the colony count above which counting error becomes unacceptable, typically due to overcrowding or antibiosis. Strict adherence to the UCL ensures the validity of the reported CFU count.
📊 Key Performance Characteristics
To ensure reliable data quality, the standard defines specific performance metrics adapted from Practice D3870. These metrics are essential for validating the method in different water matrices and for interpreting the results of the two-step membrane filter procedure.
| 📏 Characteristic | 📐 Definition from Standard |
|---|---|
| Accuracy | The proportion of the observed count to the true density of a sample. |
| Bias | The persistent positive or negative deviation of the average value of the test method from the assumed or accepted true value. |
| Precision | The degree of agreement of repeated measurements, expressed quantitatively as the standard deviation or the 95 % confidence limits of the mean. |
| Specificity | The ability of a test method to select or distinguish the target bacteria, reported as the percent of false positive and false negative results. |
| Upper Counting Limit (UCL) | That colony count above which there is an unacceptable counting error, which may be due to overcrowding or antibiosis. |
These definitions provide a consistent vocabulary for laboratories performing the analysis, ensuring that the reported data is analyzed and presented with a clear understanding of the method’s inherent limitations and strengths.
❓ Frequently Asked Questions
🔍 What is the detection limit for E. coli using ASTM D5392-24?
The detection limit for this two-step membrane filter procedure is one colony forming unit (CFU) per volume of water filtered.
💡 What types of water are validated for this test method?
According to the scope, this test method has been used successfully with temperate fresh and marine ambient waters, and wastewaters. Users must validate the method for other water types.
⚡ What is the significance of finding E. coli using this method?
E. coli is a bacterium found exclusively in the feces of humans and other warm-blooded animals. Its presence in water is a direct indication of fecal pollution and the possible presence of enteric pathogens.
📌 What are the standard’s requirements for test water quality?
The standard requires reagent water conforming to Specification D1193 for the preparation of media and reagents used in the two-step membrane filter procedure.