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Your muscles, heart, and liver run heavily on fat. But fat cannot enter the part of your cells that burns it for energy without a specific carrier molecule along for the ride. Free carnitine is the pool of those carrier molecules that are unused and ready to grab their next fatty cargo. When this number is low, your cells are running short on the very thing they need to access fat as fuel.
Of all the numbers on a carnitine panel, free carnitine is the one that most directly answers the question of whether a person is actually deficient. Total carnitine can look normal even when something is wrong, but free carnitine tells you whether there is meaningful capacity left in the system. This is the value used in newborn screening programs to catch primary carnitine deficiency, and it is the value that drops first when valproate, dialysis, or rare inherited transporter problems start to deplete you.
Carnitine in your blood comes in two forms. Free carnitine is the unbound, ready-to-work pool. Esterified carnitine (also called acylcarnitine) is carnitine that has already grabbed a fatty fragment and is in the middle of doing its job. The two added together make total carnitine. Free carnitine is reported on its own because it has unique meaning: it represents the available capacity of the system, not the inventory currently in use.
A low free carnitine value usually means one of three things. Your body is not getting enough from food and not making enough internally to keep up. Something is depleting it faster than you can replace it, like a medication or dialysis. Or, rarely, you have an inherited problem with the kidney transporter that normally pulls carnitine back out of urine, causing your body to leak it out continuously. In primary carnitine deficiency, free carnitine can drop to less than 5% of normal.
About three-quarters of your body's carnitine normally comes from food, mostly red meat and dairy. The rest your liver and kidneys make from two amino acids called lysine and methionine. Healthy omnivores rarely run low. Strict vegetarians and vegans have measurably lower plasma carnitine than meat eaters because their bodies must produce most of their carnitine from scratch, though most still stay within the normal range thanks to compensatory increases in internal production and kidney reabsorption.
True deficiency comes in two flavors. Primary carnitine deficiency is a rare inherited problem with the kidney transporter, and it shows up as a dramatically low free carnitine value alongside proportionately low acylcarnitines. Secondary deficiency is far more common and is driven by things like long-term valproate (an anti-seizure medication), kidney failure, dialysis, malabsorption from gut disease, severe undernutrition, or premature birth.
Your heart muscle stores more carnitine per gram than almost any other tissue, because it depends on fat oxidation for most of its energy. People with established heart failure tend to have lower plasma carnitine than the general population, and observational research has reported a J-shaped relationship between free carnitine and one-year outcomes after a first heart failure hospitalization, with both the lowest quartile (under 37 micromoles per liter) and the highest quartile (above about 59 micromoles per liter) linked to worse outcomes (a J-shape just means risk rises at both ends and is lowest in the middle).
Skeletal muscle is the other main storage site, which is why unexplained muscle weakness, exercise intolerance, or muscle breakdown sometimes prompts this test. Severe primary deficiency can cause progressive cardiomyopathy and muscle disease. These are uncommon, but they are treatable when caught, which is part of why measuring free carnitine is worth it for the people who genuinely need to know.
If you take valproate (Depakote, Depakene) for seizures, migraines, or mood stabilization, this is one of the few labs you should consider running. Valproate depletes carnitine through several mechanisms at once: it impairs how the kidneys reabsorb carnitine, it interferes with the body's own carnitine production, and one of its metabolites competes with carnitine for the cell-membrane transporter.
In one study of children on valproate alone, mean plasma free carnitine was about 29.9 micromoles per liter (a unit for very small concentrations), compared with about 36.8 in healthy controls and 36.7 in children on other anticonvulsants. Children on valproate plus a second anti-seizure drug showed even larger reductions, averaging about 21.4 micromoles per liter. Adults on long-term valproate alone often maintain normal plasma free carnitine because the kidneys compensate by producing more carnitine transporter, but the clinical concern is that depletion may contribute to the rare but serious complications of liver toxicity and high blood ammonia.
Reference values vary by lab and assay, so use these as orientation, not absolutes. The most widely cited cutpoint comes from the National Institutes of Health Office of Dietary Supplements and standard biochemical genetics references.
| Free Carnitine Level | What It Suggests |
|---|---|
| Above 20 micromoles per liter | Within the normal range. The pool of ready-to-work carrier molecules is adequate. |
| 20 micromoles per liter or less | Suggestive of insufficiency. Worth confirming with a repeat test and investigating the cause. |
| Less than 5% of normal (roughly under 2 to 3) | Pattern seen in primary carnitine deficiency from an inherited transporter defect. Requires specialist evaluation. |
These thresholds are drawn from published research and clinical reference labs. Your lab may use slightly different assays and cutpoints. Compare your results within the same lab over time for the most meaningful trend.
A single free carnitine measurement can be thrown off by a recent meal, acute illness, or hydration status, so any abnormal result is worth confirming with a second test. If you are starting or already taking valproate, a baseline reading and a follow-up at three to six months is a reasonable cadence, then yearly if stable. If you are on dialysis, your nephrology team likely already tracks this. If you are vegan or vegetarian and curious about your levels, a baseline and an annual recheck is enough unless you are making other major dietary changes.
Trending also matters because your number tells you whether an intervention is working. If you are taking L-carnitine because of documented deficiency, a recheck in eight to twelve weeks confirms that your free carnitine is rising back into range. If it is not moving, the supplement is not solving the problem and something else is going on.
Free carnitine is sensitive to short-term inputs. A meat-heavy meal in the hours before your draw can transiently push the number up. A long fast can shift it because your body is burning more fat than usual. Acute illness, surgery, or recent intense exercise can also temporarily change the result. For the most reliable reading, test in a stable, fed but not heavily meat-loaded state, and avoid drawing labs during or immediately after acute illness.
Evidence-backed interventions that affect your Free Carnitine level
Free Carnitine is best interpreted alongside these tests.