Functional Medicine Testing
The best test is not the most advanced one. It is the one that answers the next important question.
Most people arrive at functional medicine testing with a list. Stool test, hormone panel, food sensitivities, mould, metals. The list is not the problem. The order is. Run in the wrong sequence, expensive tests produce findings nobody can interpret, and people spend years treating results instead of causes. Run in the right sequence, each layer narrows what the next one has to explain.
Begin reading
The order, not the menu
01 History and assessment
02 Comprehensive blood chemistry
03 Digestive function
04 Hormones
05 Organic acids
06 SIBO
07 Food sensitivities
08 Micronutrients
09 Heavy metals
10 Mould and mycotoxins
11 Environmental medicine
Each layer earns its place by answering a question the one above it could not. That is the whole logic of this page.
There is a version of functional medicine that is really just a shopping list. Someone feels unwell, a practitioner orders four specialty panels at once, and three weeks later a stack of colourful reports arrives full of arrows and asterisks. Almost every one of those reports will contain something abnormal, because almost every one of them is designed to. What nobody can tell you from that stack is which finding is driving the problem, which is a consequence of it, and which would have corrected itself once something upstream was addressed. That is not a testing problem. It is a sequencing problem.
Biology has an order. Testing should have one too.
This page explains how a functional medicine physician decides what to ask and when. It walks the eleven layers in the order they are usually run, what question each one answers, when it earns its place, why it sits where it sits, and what specifically goes wrong when it is ordered too early. It is a way of thinking rather than a catalogue, and it is written so that you can hold your own testing to the same standard, whoever orders it.
The principle
The best test is not the most advanced one. It is the one that answers the next important question.
Everything else on this page is a consequence of that sentence.
Free guide
The Testing Roadmap
The same sequence set out on this page, in a form you can take to any clinician. What belongs on a comprehensive blood chemistry panel, which markers are worth arguing for, why the order of testing matters more than the number of tests, when specialty testing genuinely earns its place, and the questions to ask before agreeing to any panel.
The Testing Roadmap
How a functional medicine physician decides what to test, and in what order.
The argument this whole page rests on
Sophistication is not the same as usefulness.
A test is useful when its result changes what you do next. That is the entire standard. By that standard a twelve dollar ferritin can be more valuable than a six hundred dollar panel, because a ferritin of eight explains the fatigue, the hair loss, the breathlessness and the restless legs all at once, and it tells you exactly what to do on Monday morning.
The reverse is also true, and it is the more expensive mistake. Advanced panels return dense results whose meaning depends heavily on context. Run one before the context exists and you get findings that are technically real and clinically unreadable. Elevated organic acid markers in someone with untreated hypothyroidism tell you almost nothing, because the thyroid is producing them. Correct the thyroid and half of them normalise on their own.
This is why order matters more than the number of tests. Each layer removes explanations, so the next layer has less to account for. Skip layers and you are not testing more thoroughly, you are testing more expensively into a wider space of possible answers.
A finding you cannot interpret is not information. It is a bill.
The same panel, two different weeks
Nothing changed except the order
The same panel, two different weeks
Run first, before anything else
Eleven markers come back flagged as abnormal
There is no way to rank which one matters
Six supplements get started at the same time
Nobody can tell which one, if any, helped
A retest three months later is just as ambiguous
Run after blood chemistry
Four of the eleven findings are already explained
Three of those were thyroid driven and have corrected
One genuine question remains, and it is clearly framed
The panel did not change. The context it landed in did. This is the difference between a test that informs a decision and a test that generates homework.
The cost is not only financial
Testing out of order has a predictable failure pattern.
It rarely fails loudly. It fails by producing a plausible story that sends someone in a direction they will spend the next eighteen months walking. The money matters, but the time and the false confidence usually matter more.
Two ways to spend the same budget
Specialty testing first
Many findings arrive with no hierarchy between them
Several changes get started at the same time
Cause and effect become unreadable
Retesting repeats the same confusion
Cost rises but clarity does not
Confidence builds in a story that may be wrong
The actual driver is still sitting there untouched
Foundations first
Fewer findings, and they can be ranked
One change at a time, so the effect is measurable
Cause and effect stay visible throughout
Retesting answers a question that was actually asked
Later tests end up cheaper and fewer
Confidence that survives scrutiny
The real driver surfaces sooner
A wrong explanation is more expensive than no explanation, because people stop looking once they think they have one. Order is not bureaucracy. It is what makes results mean something.
The hierarchy, layer by layer
Each layer answers a question the one above it could not.
These first six carry most of the weight. They are where sequencing errors are common and where they cost the most, so each one is set out in full: the question it answers, when it earns its place, why it sits where it sits, and what specifically goes wrong when it is run too early.
The question it answers. What has actually happened to this person, in what order, and what changed around each turning point. Not a symptom list. A timeline with causes attached to it.
When it earns its place. Always, and before anything is ordered. There is no clinical situation in which this layer is skippable.
Why it comes first. Because it decides which of the layers below is even worth running. Studies of general medical outpatients have repeatedly found that the history alone points to the diagnosis in roughly three quarters of cases, with examination and testing largely confirming what the conversation already suggested. Testing is a way of checking a hypothesis, and the hypothesis comes from here.
What goes wrong when it is skipped. Testing becomes a fishing expedition. Without a timeline there is no way to tell whether a finding is a cause or a consequence, and that single distinction is what most testing arguments are actually about. A gut finding in someone whose symptoms began after a course of antibiotics means something different from the same finding in someone whose symptoms began after a bereavement. The result is identical. The meaning is not.
The question it answers. How are the major systems actually running right now. Thyroid, iron, blood sugar regulation, kidney and liver function, inflammation, electrolytes, the blood count, vitamin D. The wide, cheap, well validated view.
When it earns its place. Essentially always, and early. This is the layer people are most tempted to skip because they have had blood work before and were told it was normal. Normal on a reference range and optimal are different questions, and the panel most people have had is narrower than this one.
Why it comes after history. Because the history decides which markers are worth arguing for and how the results should be read. It also decides what counts as a meaningful change, which matters more than any single value.
What goes wrong when it is skipped. This is the most expensive mistake on the page. A large share of the abnormalities that show up on specialty panels are downstream of something visible here. Untreated hypothyroidism, iron deficiency, poor glucose regulation and active inflammation all distort organic acids, hormone results and gut findings. Run the specialty panel first and you will treat the shadow instead of the object. Comprehensive Blood Chemistry
The question it answers. Is food actually being broken down and absorbed, is the barrier holding, and is the microbial community disturbed in a way that matters.
When it earns its place. Digestive symptoms that persist, systemic problems that blood chemistry did not explain, a history of repeated antibiotic courses, or blood findings that point upstream to absorption such as low ferritin, low B12, low protein or unexplained inflammation.
Why it comes after blood chemistry. Because blood chemistry frequently tells you the gut is the problem before any stool sample is taken, and because correcting what blood chemistry found often changes the gut picture on its own. Thyroid function alone changes transit time, stomach acid production and bile flow.
What goes wrong when it is run too early. Comprehensive stool panels report something abnormal in almost everyone. Low diversity, a marker slightly out of range, an organism of uncertain significance. Without blood context there is no way to judge whether any of it is driving anything, and people end up on long antimicrobial and supplement protocols aimed at findings that were never the problem. Digestive Function Testing and Gut Health
The question it answers. Are the signals that regulate energy, mood, cycle, sleep and the stress response being produced, delivered and cleared appropriately, and what does their pattern across a day or a cycle look like.
When it earns its place. Cycle problems, fertility, perimenopausal change, sleep that fails at a consistent time, mood or energy that follows a rhythm rather than a trigger, and symptoms that persist after thyroid and iron have been properly addressed.
Why it comes after the gut. Because absorption and the microbial community both influence how hormones are cleared. Bacterial enzyme activity in the gut can reactivate oestrogen that the liver had already packaged for excretion, which means a gut problem can present as a hormone problem. Treating the hormone in that situation is treating the symptom.
What goes wrong when it is run too early. Cortisol curves and sex hormone results read very differently in the presence of untreated thyroid disease, iron deficiency or poor absorption. People are placed on hormone protocols for what was a nutrient problem, feel slightly better for a while because hormones are powerful, and lose two years. Hormone Testing and Hormone Health
The question it answers. What is happening inside cellular metabolism. Mitochondrial energy production, neurotransmitter turnover, detoxification capacity, B vitamin sufficiency at the point of use rather than in the blood, and byproducts produced by gut organisms.
When it earns its place. When the picture is still genuinely unexplained after the first four layers, particularly with fatigue, mood, cognitive symptoms or neurological complaints that do not fit what has been found so far.
Why it comes here. Organic acids are a downstream readout of everything above them. They are shaped by thyroid function, nutrient status, gut organisms and inflammation, which makes them enormously informative once those are known and close to meaningless when they are not.
What goes wrong when it is run too early. This panel returns a long list of flagged markers on almost anybody, and each flag comes with a suggested nutrient. Run first, it generates a supplement regimen aimed at consequences. Run fifth, it usually generates one or two questions worth pursuing, because most of the noise has already been accounted for. Organic Acids Testing
The question it answers. Is there bacterial overgrowth in the small intestine specifically, as opposed to a disturbance further down where bacteria are supposed to live.
When it earns its place. Bloating that arrives shortly after eating rather than at the end of the day, visible distension, reflux that does not respond to the usual measures, alternating bowel habit, or findings from the previous layers that hint at overgrowth.
Why it comes after the gut and metabolic layers. Because overgrowth is nearly always secondary to something else. Impaired motility, low stomach acid, structural change, previous food poisoning, adhesions, thyroid disease, or long term acid suppressing medication. The test tells you overgrowth is present. It does not tell you why the small intestine stopped clearing itself, and that is the question that determines whether treatment holds.
What goes wrong when it is run too early. A positive result is treated, symptoms improve, and the overgrowth returns within months because nothing upstream changed. People go through three and four rounds of this before anyone asks about motility. SIBO Breath Testing
The specialty layers
These five are powerful, and they are the easiest to order too early.
Each of these answers a real question and each has a genuine place. What they share is that their results are heavily shaped by everything above them, which is why they sit at the bottom rather than at the top. Treated in shorter form here, because the sequencing principle is the same one you have already read five times.
The question. Is the immune system reacting to specific foods, and is that reaction contributing to what this person is experiencing.
Why it sits here, and the early ordering mistake. A permeable, inflamed gut barrier produces reactivity to whatever is being eaten most often, which means the test frequently measures the state of the barrier rather than a fixed property of the food. Run it before the gut has been assessed and people remove twenty foods, feel better briefly because the total load dropped, then find the list growing. Addressing the barrier first often shrinks the list without removing anything permanently. Food Sensitivity Testing
The question. Are the raw materials actually present inside cells, as opposed to circulating in serum where they are easier to measure and less informative.
Why it sits here, and the early ordering mistake. Nutrient status is downstream of intake, digestion, absorption and demand. A deficiency found before the gut has been looked at tells you a number without telling you why, and supplementing it usually works until it is stopped. Run after digestion has been assessed, the same result becomes actionable, because you know whether you are correcting a shortfall or plugging a leak. Micronutrient Testing and Nutrient Deficiencies
The question. Is there a meaningful body burden of lead, mercury, cadmium, arsenic or aluminium, and is it plausibly contributing.
Why it sits here, and the early ordering mistake. Interpretation depends on nutrient status and on how well the body is clearing anything at all, both of which are established by earlier layers. It also depends on knowing whether an exposure is realistic, which comes from the history. The specific hazard here is that mobilising metals in someone whose nutrient status, gut barrier and clearance pathways have not been addressed can make them considerably worse. Sequence protects people here, not just budgets. Heavy Metal Testing
The question. Has there been meaningful exposure to water damaged building material, and is the body carrying mycotoxin load as a result.
Why it sits here, and the early ordering mistake. Results are strongly influenced by antioxidant reserves and clearance capacity, so the same exposure produces very different numbers in two different people. It also matters enormously whether the exposure is current or historic, which is a history question rather than a laboratory one. Ordered too early, it produces a diagnosis people build their identity around while the ongoing exposure in the bedroom goes unaddressed. Mould and Mycotoxin Testing
The question. What is the total chemical and environmental load this person is living inside, and which parts of it are modifiable.
Why it sits last, and the early ordering mistake. Not because it is unimportant. It is often the most important thing in the room. It sits last because it is the broadest and least specific layer, and because acting on it well requires knowing which systems are already struggling. Ordered first, it produces an overwhelming list and very little direction. Ordered last, it explains why the previous ten layers kept finding the same pattern. Environmental Medicine
On the order itself
The sequence is not a rule about which tests are better. It is a rule about which results can be read.
Any layer can be moved when the history demands it. The history is the only thing that outranks the order.
Why the second layer carries so much weight
A systems map, with blood at the centre.
A properly built blood chemistry panel is not one test. It is a simultaneous read on ten systems that all influence each other, which is why it explains so much of what specialty panels later flag. It is also the cheapest layer on this page by a wide margin, and the most widely available.
A systems map, with blood at the centre
Comprehensive blood chemistry reads all of these at once
Thyroid function
Iron status
Blood sugar regulation
Inflammation
Liver function
Kidney function
Vitamin D
The blood count
Electrolytes
Lipids and metabolism
Ten systems, one blood draw, and the least expensive layer on this page. This is why so much of what specialty panels flag turns out to be downstream of something visible here.
There is a second reason this layer matters, and it is the one people find hardest to hear. Most standard panels are read against reference ranges, which describe the spread of results in a laboratory population rather than the range associated with feeling well. A result can sit inside the reference range and still be a long way from where it should be for that person. The panel is not wrong. It is answering a different question from the one being asked. Comprehensive Blood Chemistry covers the difference in detail.
The failures are predictable
Twelve ways testing goes wrong, and none of them are the laboratory.
These are ordering and interpretation errors rather than laboratory errors. The samples are usually processed correctly. What goes wrong happens before the sample is taken and after the result comes back.
Starting at layer nine
Specialty panels ordered before anything foundational is known. The results are real and unreadable, because the context that gives them meaning has not been established yet.
Ordering everything at once
Four panels in the same week feels thorough. It removes the one thing that makes testing interpretable, which is knowing what changed between one layer and the next.
Treating normal as optimal
A reference range describes a laboratory population. It was never designed to describe the range associated with feeling well, and the two are not the same conversation.
Treating optimal as a target
The opposite error, and just as common. Chasing every marker to the middle of an optimal band produces long supplement lists and very little change in how anyone feels.
Mistaking a consequence for a cause
Almost every specialty panel flags downstream effects. Without a timeline there is no way to tell which finding started the problem and which is simply reporting it.
Reading one marker in isolation
Single markers rarely mean much on their own. Patterns across several markers mean a great deal, which is why a panel beats a test and a sequence beats a panel.
Changing six things at once
If six things change together and the person improves, nobody knows what worked. The next flare has no map, and the whole exercise has to be repeated.
Testing without a decision attached
Before ordering anything, the question is what will be done differently depending on the result. If the answer is nothing, the test is expensive curiosity.
Retesting too soon
Most meaningful biological change takes eight to twelve weeks to show up, and some takes considerably longer. Retesting at four weeks usually measures noise and buys anxiety.
Never retesting at all
The other failure. Without a follow up measurement there is no way to know whether the intervention did anything, and the plan quietly becomes permanent by default.
Testing during an acute event
Infection, injury, a crash diet, a recent course of steroids or a genuinely terrible month all distort results. The panel is accurate and the timing makes it uninformative.
Building an identity around a result
The most human one. A dramatic finding gives a name to years of feeling unwell, which is a relief. It also stops the search, and quite often the real driver is still upstream of it.
In practice, not in theory
How an evaluation actually runs.
The eleven layers describe the logic. This describes what the process looks like from the inside, including the parts that are not tests at all and the point at which the ordering stops.
Use this on anyone, including me
Before you agree to any panel, ask these.
These are the questions a good clinician will welcome, because they are the questions they have already asked themselves. If a panel cannot survive them, it probably should not be ordered yet.
What question is this test answering
What will we do differently depending on the result
What have we already established that makes this the next step
What would this result look like if the problem were something else
Is anything on this panel already answered by blood work I have had
Could an untreated thyroid, iron or blood sugar problem distort this result
How reliable is this particular test, and how is it validated
What does it cost, and is that the best use of that amount right now
If it comes back normal, what does that actually rule out
If it comes back abnormal, what happens on Monday
When would we retest, and what would count as improvement
What is the plan if we find nothing at all
There is also a situation where testing is not the right first move at all. Unexplained weight loss, blood where it should not be, a lump that is new, chest pain, severe or sudden headache, fever that will not settle, night sweats, a change in bowel habit lasting more than a few weeks, or any symptom that is escalating quickly. These need medical assessment now, not a panel in three weeks. Functional medicine testing is for understanding why a system is drifting. It is not a substitute for investigating something that may be acute, and no result on this page should ever delay that.
Free guide
Take the sequence with you
The Testing Roadmap sets out the same order, the markers worth arguing for on a comprehensive panel, the difference between a reference range and an optimal range, and the questions above in a form you can hand to any clinician. Educational, not a diagnosis.
Own your biology
You are allowed to ask why this test, and why now.
Testing has become something that happens to people rather than something they participate in. A panel is suggested, a card is charged, results arrive, and a plan is handed over. At no point does anyone explain what question was being asked or what would have counted as an answer. That is not a criticism of any individual clinician. It is what happens when the menu replaces the reasoning.
You can change that with a single sentence. What question is this answering, and what will we do differently depending on the result. Ask it every time. A clinician who has thought it through will be glad you asked, because it means you will actually act on what comes back. If the answer is vague, that is information too, and it is worth more than the panel.
The aim is not fewer tests. It is fewer wasted ones, and far more confidence in the ones you do run.
Order beats volume
A result you cannot interpret is a bill
History outranks the panel
Normal and optimal are different questions
One change at a time, so you can read it
Finishing the evaluation is allowed
Bring the results you already have.
No pressure, and nothing to buy. Bring any blood work from the last two years even if you were told it was normal, any specialty panels you have already paid for, the medications and supplements you are currently taking, and a rough timeline of when things changed. Quite often the most useful hour is spent reading what already exists rather than ordering anything new.
Common questions
Questions about testing.
Short, plain answers to what people ask most.
Why start with blood work?
What is the difference between a reference range and an optimal range?
Can normal blood work miss problems?
Should I order my own labs?
Do I need fasting labs?
Can I skip blood work and go straight to specialty testing?
When should I do stool testing?
Is the DUTCH test better than blood testing?
Are food sensitivity tests accurate?
When should I test for mould?
How often should I retest?
What if I have a limited budget, where do I start?
