Functional Lab Testing: Categories and Their Evidence Base
Level 2, Intermediate
In brief
Functional laboratory tests range from well validated measures, such as faecal calprotectin, faecal elastase, apolipoprotein B and lipoprotein(a), to tests with partial validation, such as breath tests and some hormone profiles, to tests with little or no clinical validation, such as IgG food panels, hair mineral analysis, provoked urine metals and urine mycotoxins. Rating each category by analytical validity, clinical validity and clinical utility helps practitioners order fewer, better tests.
An earlier article explained what biomarkers are and how to judge whether a test is useful. This article applies that framework category by category to the tests most commonly used in functional practice. Its purpose is practical: to give practitioners a quick reference for deciding which tests deserve a place in their practice, which should be used selectively, and which are best avoided.
The framework in brief
Each category is judged on 3 questions. Analytical validity: does the test measure what it claims, accurately and reproducibly? Clinical validity: does the result reliably reflect the clinical state it is meant to indicate? Clinical utility: does acting on the result improve decisions or outcomes? Many functional tests pass the first question, some pass the second, and fewer pass the third.
Category ratings
| Category | Well validated components | Partially validated or context dependent | Not validated for clinical use |
|---|---|---|---|
| Stool testing | Faecal calprotectin for intestinal inflammation; faecal elastase for pancreatic insufficiency; pathogen PCR and C. difficile testing | Some markers of digestion and fat malabsorption | Microbiome composition profiles used to diagnose dysbiosis and guide treatment |
| Breath testing | Urea breath test for Helicobacter pylori | Glucose breath tests for SIBO in selected patients; lactose breath tests | Lactulose breath tests used broadly in people with bloating |
| Lipid and cardiovascular markers | Apolipoprotein B, lipoprotein(a), non HDL cholesterol, hsCRP in risk assessment | LDL particle number by specialised methods | Numerous proprietary cardiovascular panels without outcome data |
| Glucose and insulin | HbA1c, fasting glucose, oral glucose tolerance test | Fasting insulin and HOMA IR for trends; continuous glucose monitoring outside diabetes | Proprietary metabolic scores without validation |
| Nutrient status | Ferritin, B12 with methylmalonic acid, folate, 25 hydroxyvitamin D, iodine at population level | Red cell magnesium, zinc with inflammatory markers, omega 3 index | Lymphocyte based intracellular micronutrient panels |
| Hormones | Serum TSH and free T4; morning testosterone; late night salivary cortisol for Cushing screening | Urinary hormone metabolite panels for research insight | Salivary diurnal cortisol profiles to diagnose adrenal fatigue; salivary sex hormone testing to guide compounded hormones |
| Organic acids | Specialist diagnosis of inherited metabolic disorders | Selected markers such as methylmalonic acid | Broad panels interpreted for general wellness |
| Food reactions | Specific IgE with supportive history; coeliac serology | Breath tests for carbohydrate malabsorption | IgG and IgG4 food panels; kinesiology; electrodermal testing |
| Toxic exposures | Blood lead; blood or urine mercury by form; speciated urine arsenic; urine cadmium | Selected occupational biomonitoring | Hair mineral analysis; provoked urine metals; urine mycotoxins for mould illness |
| Genetics | HFE, familial hypercholesterolaemia genes, coeliac HLA to exclude, pharmacogenomics for specific drugs | Polygenic risk scores | Most consumer nutrigenomic panels |
| Oxidative stress and immune panels | Standard blood count and immunoglobulins where indicated | Research markers of oxidative stress | Commercial oxidative stress and broad immune profiling panels |
5 questions before ordering
- What is the clinical question? The history and matrix should generate a hypothesis the test can confirm or refute.
- Is there a validated test that answers it? Start with the most validated option.
- What will change? If the plan would be the same whatever the result, the test adds cost without benefit.
- What are the risks of false positives and incidental findings? Large panels generate abnormal results by chance.
- Is the cost proportionate? Many functional tests are paid for by patients, and costs accumulate quickly.
Why large panels mislead
If each marker on a panel has a reference range covering 95 percent of healthy people, the chance of at least one abnormal result rises quickly with the number of markers tested. A panel of 40 independent markers will produce at least one out of range result in most healthy people. Without a clear clinical question, these results invite over interpretation, further testing and unnecessary treatment.
Professional and ethical considerations
- Financial interests: practitioners who profit from selling tests or the supplements recommended by their results face a conflict of interest that should be disclosed and managed.
- Laboratory quality: tests should be performed by accredited laboratories. Some specialised tests are labelled for research use and are not validated for diagnosis.
- Informed consent: patients should understand what a test can and cannot show, its cost, and the possibility of uncertain or incidental results.
- Scope of practice: ordering and interpreting certain tests may be restricted to particular professions in some jurisdictions.
Using partially validated tests well
Tests in the middle column are not useless. A glucose breath test can support a diagnosis of SIBO in a patient with risk factors; fasting insulin can help track insulin resistance over time in the same laboratory; continuous glucose monitoring can be an educational tool. The key is to use them for defined purposes, interpret them cautiously and avoid building extensive treatment plans on them alone.
An illustrative case
A 39 year old woman with fatigue and bloating presents with results from several functional tests costing a considerable sum: a comprehensive stool profile reporting dysbiosis, an IgG food panel listing numerous foods, a salivary cortisol profile labelled as adrenal fatigue, an organic acids test suggesting several nutrient deficiencies and a hair mineral analysis. She has been recommended many supplements.
The practitioner reviews the results using the framework. The stool profile included a normal calprotectin and elastase, which are useful. The rest offered little reliable guidance. Her history suggests irritable bowel syndrome and heavy menstrual bleeding. Validated tests, a blood count, ferritin, coeliac serology and thyroid function, reveal iron deficiency. Treating the iron deficiency and a structured approach to her irritable bowel syndrome improve her symptoms. 2 useful results had been buried among many that were not.
Fewer, better tests
Functional practice is sometimes judged by the tests it orders. The best practitioners are recognised instead by the precision of their questions and the restraint of their testing. A clear framework for rating tests helps practitioners use laboratory medicine as it should be used: to answer questions that matter.
Questions about functional tests
Which functional tests are well validated?
Which tests are not validated?
Why can large test panels mislead?
References
- FDA NIH Biomarker Working Group. BEST (Biomarkers, EndpointS, and other Tools) resource. Silver Spring and Bethesda: FDA and NIH; 2016.
- Stapel SO, Asero R, et al. Testing for IgG4 against foods is not recommended as a diagnostic tool: EAACI Task Force Report. Allergy. 2008;63(7).
- Seidel S, Kreutzer R, Smith D, et al. Assessment of commercial laboratories performing hair mineral analysis. JAMA. 2001;285(1).
- American College of Medical Toxicology. Position statement: post chelator challenge urinary metal testing. Journal of Medical Toxicology. 2010;6(1).
- Cadegiani FA, Kater CE. Adrenal fatigue does not exist: a systematic review. BMC Endocrine Disorders. 2016;16(1).
- Mach F, Baigent C, Catapano AL, et al. 2019 ESC/EAS guidelines for the management of dyslipidaemias. European Heart Journal. 2020;41(1).
Continue your learning
Choosing tests that change decisions
Choosing the right test, and knowing which results genuinely change decisions, is taught throughout IIFM’s 6 month Functional Medicine and Clinical Nutrition program, module by module.
This article is for the education of healthcare professionals. It is not a substitute for individual medical advice, diagnosis or treatment, and does not change any practitioner’s scope of practice. Regulation and guidelines differ between countries.