BACKGROUND: Ketonuria (on standard urine testing) is a frequent finding in children presenting to emergency departments. With the advent of hand-held ketone meters, blood ketone levels can now be rapidly quantified. HYPOTHESIS: Point of care testing (POCT) of blood ketone levels could provide clinically useful information on severity of illness in children and risk of hospital admission. METHODS: A prospective study using POCT of blood ketone levels in a convenience sample of children <13 years old, with a typical case mix of medical problems. FINDINGS: 186 children were studied. The range of ketone levels varied widely among this study population depending on the presenting complaint. Higher levels were noted in those presenting with anorexia or vomiting and fever. The median ketone level of the total study population was 0.2 (range 0-6.0, interquartile range 0.1-0.9) mmol/l. Ketone levels correlated poorly with discharge destination and duration of admission. However, receiver-operator characteristics for ketones as a predictor of admission were comparable to Pediatric Risk of Admission scores (area under the curve 0.64 and 0.72, respectively) and may represent an independent risk factor for admission. A ketone level >1.2 mmol/l has a positive predictive value of 66.7% for admission. Ketone levels correlated well with decreased oral intake (R2 = 0.25; p<0.001). CONCLUSIONS: A strong association was found between ketone levels, decreased oral intake and fever. Although ketone levels do not correlate well with more traditional markers of illness severity, they can help to predict the requirement for admission to hospital when interpreted in the context of the presenting illness. They may have applications in both the emergency department and primary care settings. Further prospective testing is required to validate these findings.