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Well, it’s been a while, but let’s talk about lab tests again. I’ve chosen Ketones this time because they’re so simple.
Background: The development of ketosis requires changes in adipose tissue and the liver. Ketone bodies are made from fatty acids from said adipose stores. Normally, they are taken up by the liver and are re-esterified into triacylglycerides and stored or re-entered into the circulation as LDLs. Low insulin leads to the increase in circulating fatty acids. The relative increase in glucagon increases ketone production, leading to accumulation of acetoacetate in the blood. Some decarboxylate into acetone, but the majority is converted into B-hydroxybutyrate.
Excessive ketone formation results in increased blood and urine concentrations. This process can be observed in conditions with decreased availability of carbohydrates (vomiting/starvation) or decreased use of carbs (diabetes, alkalosis, glycogen storage diseases). Diabetes mellitus and alcoholims are common causes of ketoacidosis a metabolic condition where the pH of the blood is too low, which can be fatal.
Testing: Obviously, quantitating blood ketones is more accurate than urinary ketones, since the amount excreted doesn’t always reflect the amount in circulation. But urinary ketones is much more convenient and is generally used to monitor diabetics during suspected diabetic ketoacidosis (DKA). The test methods don’t measure all three of acetoacetate, acetone, and B-hydroxybutyrate.
Urinary Ketones: Done with acetest or ketostix, both semiquantitative, neither of which measure B-hydroxybutyrate.
Acetest: This is a tablet based nitroprusside reaction and the tablet contains glycine, sodium nitroprusside, disodium phosphate, and lactose. Acetone/acetoacetate in the presence of glycine reacts with nitroprusside to make a lavender colored complex. Disodium phosphate and lactose enhance color formation. The kit also contains a color chart to help you determine, roughly, the specific concentration of ketones based on the intensity of the lavender.
Ketostix work on the same principle, but instead of a tablet, it is all part of a stick you dip into the sample. Color formation is faster (15 seconds as opposed to 10 minutes).
Ketone levels are also present on the urine dipsticks in a routine urinalysis, but they’re less sensitive so we don’t use that result here. This is partly because they don’t contain glycine, so the acetone pretty much doesn’t react at all.
Serum Ketones: Ketostix and acetest can still be used in blood samples, but there is one other option that can be used:
B-hydroxybuturate determination: B hydroxybutyrate (with the help of the cofactor NAD+) is converted into acetoacetate with a dehydrogenase enzyme. The NADH produced in the reaction can be measured either as itself at 340nm or coupled to another reaction to produce a colour and measured that way.
When it comes to the nitroprusside reactions, it is important to take note of interferences. You will get false positives if the patient sample contains high levels of drugs like L-dopa and metabolites or captopril, and, being a color reaction, samples that are highly pigmented can be hard to read. Because nitroprusside has no reactivity with B-hydroxybutyrate, it is important to remember that a negative reaction doesn’t always rule out ketosis (in severe DKA, the levels of B-hydroxybutyrate can be up to 6 times as high as the acetoacetate).
That aside, ketone levels, in combination with blood/urine glucose, is most often used to assess the severity of diabetic ketoacidosis, or in the investigation of acute alcohol abuse, and starvation. But ketosis can also be seen in people who exercise a lot, people on Atkins, pregnant ladies, and people under high stress and not be a matter of concern.