In short
- Cancer cannot be reliably seen in blood. In people without symptoms, tumour markers mostly give false positives — that is why they are not suitable for screening.
- Stress cannot be measured. Morning cortisol is a screening test for adrenal gland disease, not a measure of how overloaded you are. “Adrenal fatigue” is not a diagnosis.
- “Food intolerance” tests based on IgG antibodies measure a normal response to food you eat — professional societies advise against them.
- Blood does not see sleep apnoea, depression, burnout, pain or sleep quality.
- Blood does not replace national screening programmes (in Slovenia: SVIT, DORA, ZORA) or imaging.
- This is not an argument against testing — it is an argument for testing the right things.
Every provider of blood tests has a list of what it can measure. You will rarely find a list of what it cannot. That list is what separates an honest offer from selling fear, so we publish it in full.
1 · Can cancer be seen in blood?
Not reliably. Tumour markers (CEA, CA 125, CA 19-9, CA 15-3, AFP) were developed for monitoring a disease that is already known. They were not made for finding cancer in healthy people. In people without symptoms, the great majority of raised values are false positives. Further tests, procedures and months of worry follow, while studies have shown no survival benefit. That is why they do not belong in a screening panel for healthy people. The one exception, with clear conditions, is PSA as an informed choice for men between 50 and 70.
What to do instead: respond to the invitations of the national screening programmes — in Slovenia SVIT (bowel), DORA (breast) and ZORA (cervix). Studies have proven that these methods reduce mortality.
2 · Is there a test for stress?
No. Cortisol in blood has a narrow but real role. The morning value is a screening test for a lack of adrenal hormones. Late-evening cortisol in saliva is a screening test for their excess (Cushing's syndrome). Neither of these measurements tells you how overloaded you are. “Adrenal fatigue” is not a medical diagnosis and is not recognised by any professional organisation.
3 · Can food intolerance be measured?
Not with IgG antibodies. These antibodies form through normal exposure to the food you eat — so a high result mainly means that you eat that food. The European and American allergy societies (EAACI, AAAAI) advise against such tests. That is why we do not recommend them.
What to do instead: if an allergy is suspected, specific IgE or a skin test with an allergist. If lactose intolerance is suspected, a breath test. With digestive problems plus iron deficiency, a test for coeliac disease.
4 · Can blood show why I sleep badly?
No. Obstructive sleep apnoea is diagnosed with a sleep study, and insomnia through your medical history. Blood can rule out a few causes that make sleep worse, such as the thyroid or iron deficiency in restless legs. But it cannot assess sleep itself.
5 · Is there a test for burnout or depression?
No. No blood test makes these diagnoses. Blood is useful because it rules out physical causes that mimic them (thyroid, anaemia, blood sugar, vitamins). That is an important step, but not a diagnosis.
6 · Does a blood test replace a visit to the doctor?
No. Clinical judgement combines medical history, examination and tests. A blood test without context can mislead in both directions: it reassures when it shouldn't, or frightens over a harmless deviation.
7 · Does it show the full state of the organs?
Partly. Liver enzymes detect cell damage, not the structure of the liver. Creatinine estimates kidney function, not the shape of the kidneys. For structure you need imaging (ultrasound, CT, MRI).
8 · Can we measure “toxicity” or the need for a detox?
No. We can measure specific substances when there is a specific suspicion — lead, mercury, cadmium after occupational or dietary exposure. There is no general “toxin” test, because “toxin” is not defined. “Detox” programmes based on it have no professional basis.
9 · Can blood tell me which diet suits me?
No. There is no blood test that predicts whether a low-carb or a Mediterranean diet will suit you better. Blood does, however, show the effect of a change: triglycerides, HbA1c, ApoB and liver enzymes move within three months of a good change.
10 · Is a normal report proof that I am healthy?
No. A normal report means that on that day, for those tests, there were no deviations. Many diseases do not show up in standard tests. That is why we always read a report together with symptoms and risks. It is also why we never claim that “the check-up shows you are healthy”.
Why we publish this. Because the line between a useful test and selling fear runs through this list. Each of the ten answers above is a promise we refuse to make. It is also the reason you can trust the rest of the numbers in your explanation.
Frequently asked questions
Why don't you recommend tumour markers for screening?
Because in people without symptoms they mainly cause false positives and unnecessary procedures, and studies have shown no survival benefit. The exception is PSA as an informed choice for men aged 50–70.
Is there any blood test for early cancer detection?
For some special cases, yes (for example AFP in monitoring people with cirrhosis), but not as general screening in healthy people. For that there are national programmes with proven benefit.
Why don't you recommend food intolerance tests?
Because they measure a normal response to the food you eat, not intolerance. EAACI and AAAAI state this explicitly.
So what is a blood test good for?
It detects common and silent things: iron deficiency, thyroid problems, prediabetes, fatty liver, kidney impairment, heart risk and vitamin deficiencies. All of these can be corrected if caught early.
Will you tell me when a value cannot be assessed?
Yes. When a value has no solid scientific basis for an assessment, the app says so instead of giving a rating.
Sources: UKCTOCS 2021 (Menon et al., Lancet) — CA-125 screening with no mortality benefit · ESMO/NCCN — tumour markers for monitoring, not screening · EAU 2024 and USPSTF 2018 — PSA · EAACI 2008 (Stapel et al.) and AAAAI — IgG food tests · Endocrine Society 2008 and 2016 — cortisol, Cushing's, adrenal insufficiency · Cadegiani and Kater 2016 — “adrenal fatigue” as a myth · AASM — diagnosis of sleep apnoea · The SVIT, DORA and ZORA programmes.
