The degree of malnutrition is usually measured in terms of weight, expressed in standard deviations from the mean of the relevant reference population. When one or more previous measurements are available, lack of weight gain in children, or evidence of weight loss in children or adults, is usually indicative of malnutrition. When only one measurement is available, the diagnosis is based on probabilities and is not definitive without other clinical or laboratory tests. In the exceptional circumstances that no measurement of weight is available, reliance should be placed on clinical evidence.

If an observed weight is below the mean value of the reference population, there is a high probability of severe malnutrition if there is an observed value situated 3 or more standard deviations below the mean value of the reference population; a high probability of moderate malnutrition for an observed value located between 2 and less than 3 standard deviations below this mean; and a high probability of mild malnutrition for an observed value located between 1 and less than 2 standard deviations below this mean.