When interpreting biochemical test results in paediatric patients, consider a number of issues that are associated with and specific to childhood. These include the age of the patient, which may vary from 14 weeks prematurity to more than 18 years, and their body weight, which may range from 500 grams to more than 100 kg. Body size is also a factor in certain situations and is of special concern in the current epidemic of childhood obesity. Children are not miniature adults, however as they age their biochemistry becomes more like that of adults. Added to these patient factors are the effects of the collection process on the blood sample, the method used to analyse the sample, the source of the reference range quoted with the result, and the interpretation placed on the result by laboratory staff.