Urine Ketones: Acetest

The Acetest is performed whenever a positive result is obtained for ketones on the dipstick analysis. Ketones (primarily acetoacetate) are detected by their reaction with sodium nitroprusside to form a purple complex. This color change on a dipstick can be detected visually by a human observer or "read" by a machine, such as the Criterion at Cornell University. Although the machine is more accurate for detecting subtle changes in color than the human eye, it is not perfect and false positive results for ketones still occur because the color change is quite subtle. The Acetest for ketones is a tablet version of the dipstick and forms a distinct purple color and contains lactulose, which makes the color change easier to read. Thus, we confirm any positive Criterion ketone reactions with the Acetest tablet test, this result being a more accurate assessment of urinary ketones. Hence, if a positive reaction (regardless of the value) is obtained on the dipstick, but the Acetest is negative, the result for ketones should be interpreted on the basis of the Acetest, and not the dipstick result, i.e. the result for ketones is negative (and the dipstick is a false positive value). The Acetest is useful for semi-quantitatively measuring ketones in other fluids, such as plasma, serum and milk.

The advantage of the Acetest is that observation of the true color reaction is less affected by the inherent color of the urine itself and the color change is less subtle than on the dipstick. It is, therefore, useful in confirming or refuting apparently positive reactions on the dipstick in cases where the urine sample is deeply colored.
sorry Purple color around the tablet is a positive result.

Cornell University