This International Standard gives guidelines for the evaluation of uncertainty in quantitative microbiological analyses based on enumeration of microbial particles by culture. It covers all variants of colony count methods and most probable number estimates. Two approaches, the component (also known as bottom-up or step-by-step) and a modified global (top-down) approach are included. The aim is to specify how values of intralaboratory operational variability and combined uncertainty for final test results can be obtained. The procedures are not applicable to methods other than enumeration methods. NOTE 1 Most annexes are normative. However, only the annexes relevant to each case are to be applied. If the choice is the global approach, then all normative annexes that belong to the component approach can be skipped and vice versa. NOTE 2 Pre-analytical sampling variance at the source is outside the scope of this International Standard, but needs to be addressed in sampling designs and monitoring programmes. NOTE 3 The doubt or uncertainty of decisions based on the use of analytical results whose uncertainty has been estimated is outside the scope of this International Standard. NOTE 4 The extra-analytical variations observed in proficiency tests and intercalibration schemes are also not detailed in this International Standard, but it is necessary to take them into consideration in analytical control. The use of intercalibration data in uncertainty estimation offers the possibility for the bias between laboratories to be included (Nordtest Report TR 537[12]).