Blood draw
Carnitine Differentiation
Carnitine fraction profile; fatty acid metabolism.
€ 116,00
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Vitamins, minerals and amino acids · Blood
AC/FC ratio
Calculated ratio between bound and free carnitine; a hint as to whether a relatively large amount of carnitine is occupied.
At a glance
- Measured in blood
- In 1 test panel
- No special preparation
Explanation
AC/FC ratio: what is measured?
The AC/FC ratio is not measured but calculated. The laboratory divides acylcarnitine (AC, the carnitine that is bound to fatty acid or amino acid residues) by free carnitine (FC, the part that is still available). The idea behind it is that in a disorder of fatty acid breakdown, a lot of carnitine becomes occupied by built-up fatty acid residues, while the free part falls. The ratio puts those two values into one number.
What can it detect?
- A sign that a relatively large amount of carnitine is bound, which may fit a functional carnitine deficiency
- Rarely: a sign of an inherited disorder of fatty acid breakdown, for which the full acylcarnitine profile is decisive
Result
What does the result mean?
AC/FC ratio too high
A high ratio means that relatively much carnitine is bound and little is free. This is most often caused by prolonged fasting, being ill, heavy exercise or a low-carbohydrate or ketogenic diet: the body then burns a lot of fat. Certain medicines, such as valproic acid, and kidney dialysis also raise the ratio. A high ratio may fit a disorder of fatty acid breakdown, but the full acylcarnitine profile is decisive for that, not this one number.
AC/FC ratio too low
A low ratio means that most of the carnitine is freely available. This is the usual situation in someone who has eaten well. A low ratio causes no symptoms and has no separate meaning. The ratio can also be low when total carnitine is low; then total carnitine says more.
After an abnormal result
An abnormal ratio on its own usually does not lead to separate follow-up. The GP looks at the separate values, at how long someone had fasted, at the diet, kidney function and medicines. If there is an unexplained abnormality with symptoms of the muscles, heart or low blood sugar during fasting, referral follows to an internist (specialist in internal medicine) or a specialized center for metabolic diseases.
Reliability
How reliable is the test?
The ratio adds little beyond the separate values of free carnitine, total carnitine and the acylcarnitine profile. There are no agreed cut-off values or fixed meaning for the ratio itself. Because both underlying values depend strongly on fasting, illness, diet and kidney function, an abnormal ratio in someone without symptoms is hard to interpret. In screening for metabolic diseases, including the newborn heel prick, the pattern of individual acylcarnitines is used and not this ratio.
What can affect the result?
- Fasting, being ill, heavy exercise and a low-carbohydrate or ketogenic diet raise the ratio; how long someone had fasted largely determines the result.
- Medicines such as valproic acid and treatments such as dialysis shift both underlying values.
- Because it is a division, a small measurement difference in free carnitine can make the ratio swing sharply.
- There are no standardized cut-off values; they differ per laboratory.
Who it is for
Who is the test useful for?
Useful for
The ratio is mainly reported as part of a broad metabolic panel. In people without symptoms, an abnormal ratio says little. Anyone who wants their carnitine status assessed gets more from the separate values together with the acylcarnitine profile and the circumstances of the sample collection.
How common is it?
There are no Dutch figures on how often an abnormal AC/FC ratio occurs, because no population study has been done on it. Inherited disorders of fatty acid breakdown are rare: on average 15 to 20 children with MCADD are born each year, and they are found through the newborn heel prick.
RIVM, prenatal and neonatal screening (heel prick); no Dutch registration figure available for the ratio
Drawbacks and risks
- A raised ratio due to fasting or a low-carbohydrate diet can wrongly be read as a metabolic disease.
- An abnormal ratio with normal separate values can cause unnecessary worry or lead to unnecessary follow-up testing.
Get tested
AC/FC ratio: test panels
AC/FC ratio is part of these test panels. You order online and have your blood drawn at a collection point or collect the sample at home.
Evidence
Sources
This explanation is based on Dutch guidelines and independent information. It is general information, not a diagnosis. Discuss an abnormal result with your GP.
More about risks and resultsMore explanations
Other tests: Vitamins, minerals and amino acids
- Acylcarnitine
- Alanine
- Antibodies against intrinsic factor
- Arginine
- Calcium
- Calcium (albumin-corrected)
- Carnitine, free
- Carnitine, total
- Coenzyme Q10
- Copper
- Folic acid
- Glutamic acid (urine)