Testing mitochondrial function without a biopsy
The reference test for mitochondrial function needs a piece of muscle. Everything short of that is indirect, and read together the indirect signals answer most of the question.
Someone comes in exhausted. The thyroid is fine, the blood count is fine, iron and B12 are fine, and the exhaustion is still there every afternoon at four. The next sentence is often that their mitochondria must be struggling.
It might be true. The honest problem is that the organ you would need to sample sits inside muscle, and nobody is taking a muscle biopsy to settle a question about tiredness.
What would a biopsy have told you anyway?
Mitochondria are the structures inside cells that turn food and oxygen into usable energy. A muscle biopsy lets a laboratory measure how fast the enzyme chains inside them work, in that piece of muscle, on that day.
That is the reference standard for inherited mitochondrial disease, which is rare, serious and genetic. It is not a practical way to ask the far commoner question: is this person’s energy production running below what their life is asking of it? Everything short of biopsy is indirect, and the skill is in reading several indirect signals together rather than trusting one.
The first signal is the cheapest, and it is not a blood test at all.
What does how a person feels actually tell you?
Energy problems have a shape. Tiredness that is worse after exertion, and worse the day after rather than during, points at production rather than at sleep. So does exercise that used to feel ordinary and now takes two days to recover from.
Ask what happens on the second flight of stairs, not the first. Ask about the afternoon rather than the morning. Ask what happened to recovery after illness. These answers cost nothing and they decide whether any test is worth doing.
Tiredness that worsens after effort, and worsens the following day, is the pattern that makes energy production worth investigating.
When the story fits, ordinary blood tests carry more information than they are given credit for.
Which ordinary tests carry information?
Lactate is the most useful. When the oxygen-using pathway cannot keep up, cells fall back on a faster pathway that produces lactate, so a raised resting lactate, or one that climbs early during exertion, suggests the aerobic route is limited.
Creatine kinase, an enzyme that leaks from muscle when it is damaged, is worth having because a high value moves the question towards muscle injury instead. A full blood count and ferritin matter because anaemia and low iron reproduce the same symptoms for a different reason.
Rule out the common mimics before reaching for anything exotic: thyroid, iron, B12, sleep apnoea, depression and medication. Most people sent for mitochondrial testing have one of these instead, and finding it is a better outcome than an interesting result.
Past that point the tests become more specialised, and their claims need reading carefully.
What about organic acid and specialist panels?
Urine organic acid testing measures by-products of metabolism, some of which sit on the pathways mitochondria run. Patterns in those markers can support a picture that the story and the basic bloods already suggest.
What they cannot do is measure how well your mitochondria work. The markers are influenced by diet, gut bacteria, kidney handling and supplements, and reference ranges differ between laboratories. A result is a piece of evidence, not a verdict.
Treating a single abnormal marker on a broad panel as the diagnosis. On a panel of forty markers, some will sit outside range in a healthy person. Ask whether the pattern matches the story before acting on any one line.
The most informative test available outside a hospital is not a biochemistry panel at all.
What does testing under load add?
Energy production is only interesting when energy is being demanded, so watching what happens during exertion tells you more than anything measured at rest. A structured exercise test with gas analysis measures how much oxygen the person uses and at what point the aerobic route stops keeping up.
Repeating that test on a second day, where facilities allow it, is particularly informative in people whose symptoms worsen the day after effort, because the drop between the two days is the thing they have been describing all along.
Muscle biopsy with enzyme analysis and genetic testing is proven for diagnosing inherited mitochondrial disease. Exercise testing with gas analysis is established for measuring aerobic capacity and where it fails. Using urine organic acids or blood marker panels to judge mitochondrial function in otherwise well people is being studied; the markers are real, and their interpretation outside genetic disease is not settled. Several supplement approaches marketed for mitochondrial support have early data at best, and are honestly described as promising rather than proven.
- Start with the shape of the tiredness: during effort, or the day after?
- Exclude thyroid, iron, B12, sleep and medication before anything specialised.
- Lactate at rest and on exertion is the most useful ordinary test here.
- Read broad panels as patterns, never as single lines.
- Testing under load answers the question people are actually asking.
Where it gets interesting is the person whose ordinary tests are all normal and whose exercise test is clearly not. The Deep Dive on assessing mitochondrial function without a biopsy takes that case apart.
