Guide to blood testing.

What is blood testing?

A blood test measures a set of values — biomarkers — in a small vial of blood drawn through a brief prick at a certified lab. Each marker carries a specific signal: an organ working harder than it should, a nutrient quietly running low, inflammation simmering somewhere, a metabolic pathway drifting out of balance.

Until recently blood testing was set up to be diagnostic — you got tested when a doctor suspected something. Optimize and platforms like it shift that to preventive. You test because you want to see how your body is doing long before any complaint surfaces. A comprehensive panel measures more than forty markers in a single draw — a snapshot that, repeated over time, becomes a reliable trend line of your health.¹

Which biomarkers get measured?

A comprehensive blood test combines markers from different systems. The categories below cover most preventive panels, including the Optimize Baseline.

• Cardiovascular — total cholesterol, LDL, HDL, ApoB, triglycerides and their ratios. LDL — and even more so ApoB — predicts cardiovascular risk better than total cholesterol on its own.²
• Metabolic — fasting glucose and HbA1c (average blood sugar over ~3 months). Together they show whether insulin resistance is developing — often years before a type 2 diabetes diagnosis is on the table.³
• Liver and kidney — ALT, AST, gamma-GT, albumin, bilirubin, creatinine and eGFR. These flag overload on your two main filtering organs.
• Immune system — the full white blood cell profile (neutrophils, lymphocytes, monocytes, eosinophils, basophils) plus CRP, a sensitive marker for low-grade inflammation.
• Oxygen transport — haemoglobin, haematocrit, MCV, MCH, MCHC and red blood cells. Reads whether your blood is moving oxygen efficiently — a direct lever on energy and endurance.
• Vitamins and minerals — vitamin D, B12, folate, ferritin (iron stores), magnesium, calcium and potassium. Deficits here are often silent but weigh on sleep, recovery and mood.
• Hormones — thyroid (TSH, fT4) by default, with sex hormones (testosterone, oestradiol, SHBG) and cortisol as add-ons. The captains of energy, libido, recovery and stress response.

When is blood testing worth doing?

The obvious moment to test is when something feels off — but by then you're catching the problem late. The higher-value moment is preventive: building a baseline while you still feel well, so future measurements have something to compare against.

Three situations where a blood test pays off in practice:

1. 01
   You have no idea where you stand. A first measurement gives an honest starting point. Things often come up that you weren't feeling — ferritin slipping, ApoB drifting up, a borderline thyroid — exactly the kind of issues that are easy to course-correct early.
2. 02
   You have complaints without a clear cause. Fatigue, poor sleep, brain fog, low libido, unwanted weight gain — all of these can have multiple physiological origins. A blood test narrows the search space from 'something is off' to 'this specific system needs attention'.
3. 03
   You're actively working on your health and want to measure the effect. Strength training, a new nutrition strategy, supplementation, a tighter sleep routine — a re-test after three to six months tells you whether your interventions are actually moving the needle or whether your effort is going the wrong way.

How do you read your result?

Every result lists two numbers that matter: your value and the reference range. But reference ranges are not optimal ranges, and that distinction is the single most important step in learning to read a result.

A reference range is statistical: it's the band ~95% of people who've ever been tested at the lab fall into.⁴ That includes healthy people, but also people with quietly progressing metabolic problems. A 'normal' result therefore only says you're not in the most extreme 2.5% of the population — not that your value is optimal.

Optimal ranges are tighter and grounded in research on what minimises long-term risk or maximises function. Vitamin D between 30 and 50 ng/mL is, for example, 'normal'; between 40 and 60 ng/mL it is associated with lower respiratory-infection rates and better bone health.⁵ The same logic applies to HbA1c, ferritin, ApoB and almost every other marker.

Three tips for reading a result well:

1. 01
   Look at context, not one value. A single marker rarely says much. Ferritin at 30 ng/mL reads differently with high CRP (inflammation can falsely lift ferritin) than with low haemoglobin.
2. 02
   Compare to your own previous measurements. A value that's slowly drifting is an earlier signal than a value that suddenly falls out of range.
3. 03
   Trends beat snapshots. A re-test on the right cadence (see next section) tells you more than a single measurement ever can.

In the Optimize app each biomarker is explained in plain language, next to your value, the reference range and our optimal range, with per-marker context.

How often should you test?

How often to test depends on where you stand. A rough rule:

• Once a year — sufficient for most people without complaints who want to monitor their health. The big markers (lipids, glucose, thyroid, vitamins) move slowly.
• Every 3-6 months — when you're actively pursuing an intervention (weight loss, strength training, statin, thyroid medication, hormone therapy). A shorter cadence shows whether the intervention is working.
• Every 6-12 weeks — for specific deficits you're correcting. Iron, vitamin D or B12 supplementation should be tracked until values sit in a healthy optimum — then you stop.

What you don't want is testing without a goal or without a comparison baseline. One result in a vacuum is information; two results three months apart is a direction; four results over a year is a trend line worth making decisions on.

How to prepare for a blood draw

For most markers preparation barely matters — a blood draw is a brief, standard procedure. For a handful of specific values, timing and fasting do count.

• Fasting (8-12 hours without food, water is fine) matters for triglycerides and, to a lesser extent, glucose. LDL and HDL change little after a meal, but for consistency across labs and visits a fasted draw remains the standard.⁶
• No alcohol in the 24 hours before — alcohol temporarily lifts liver values (gamma-GT, ALT) and can skew triglycerides.
• No hard training in the 24 hours before — intense exertion briefly raises creatine kinase, CRP and ALT/AST.
• Timing of the draw matters for hormones. Cortisol peaks in the morning and falls through the day; testosterone is higher in the morning. For consistency: draw hormones between 7:00 and 10:00.

Medications and supplements you take daily should generally continue, unless your physician advises otherwise. Stopping can artificially shift your values for the measurement.

Blood testing without a GP referral

In the Netherlands you've been able to order blood tests yourself for several years now, without a referral from a GP. That's deliberate: preventive healthcare is shifting toward the individual, and an annual check-in on your values shouldn't have to start in a waiting room.

In practice this means you pick a panel via a platform like Optimize, book an appointment at one of 238+ certified draw locations and receive your result digitally in the app — typically within 24 hours. No referral, no questionnaire, no waiting list.

Important caveat: blood testing without a referral is not a replacement for regular care. At Optimize every result is reviewed by our physician through a triage system, and if something needs clinical follow-up we reach out proactively. If you suspect an acute issue yourself — unexplained severe fatigue, weight loss, fever — your GP remains the first point of contact.

What do you do with the result?

A result that sits in a PDF you never re-open is wasted time and money. The value of a blood test is in the action you pull out of it. Three layers to make your result practical:

1. 01
   Understanding. Read what each marker's value means, whether it sits in your optimal range and what it influences. The Optimize app explains every marker in plain language as standard.
2. 02
   Prioritisation. Not every out-of-range value deserves the same attention. The biggest levers are usually metabolic (HbA1c, triglycerides, ApoB), inflammation (CRP) and large deficits (ferritin, vitamin D, B12). Start there.
3. 03
   Concrete next steps. Every deviation has a well-researched set of lifestyle interventions. Elevated LDL or ApoB → reduce saturated fat, raise soluble fibre, move more.⁷ Low ferritin → increase iron intake and address what's causing the loss. Low vitamin D → supplementation plus daylight. The Optimize app translates each deviation into a tailored action plan automatically.

Book your re-test the moment you start an intervention. Three to six months later you want to see whether it's working. That turns a blood test from an isolated snapshot into a recurring feedback loop on your lifestyle.