How to read your blood test

"Normal" is not the same as optimal

Lab reference ranges are statistical: they describe where ~95% of a broad testing population falls. That population includes plenty of tired, deficient people, so sitting at the bottom edge of "normal" often just means common, not fine. Two examples where the gap is well documented:

• Ferritin (iron stores): many labs mark 12–15 ng/mL as the lower limit, yet fatigue, hair shedding, and restless legs are repeatedly reported at levels below ~50 ng/mL — especially in menstruating women.
• Vitamin D: 20 ng/mL avoids bone disease, but most expert bodies now discuss 30–50+ ng/mL for general health. See our vitamin D guide.

The practical takeaway: don't stop at "no flags on my report." Look at where in the range each value sits.

US vs. SI units — the silent confuser

The same test can be reported two ways. Israeli HMO reports often mix both. The classics:

• Vitamin D: ng/mL (US) vs. nmol/L (SI). Multiply ng/mL by 2.5 → nmol/L. "50" is great in ng/mL and borderline in nmol/L.
• Glucose: mg/dL vs. mmol/L (× 18).
• B12: pg/mL and pmol/L (× 0.738).

Always check the unit column before reacting to a number.

The markers that change decisions

• Vitamin D — 25(OH)D. The most commonly low marker in modern bloodwork, and the cheapest to fix.
• Ferritin + hemoglobin. Ferritin shows iron stores long before anemia appears in hemoglobin. Low ferritin with symptoms is worth acting on; high ferritin is a reason to investigate, not supplement. See the iron & ferritin guide.
• B12 (and folate). Especially relevant if you eat little animal protein, take metformin, or use acid blockers.
• TSH. The primary thyroid screen. Persistent fatigue + TSH drifting up is a conversation to have with your doctor before blaming your vitamins.
• HbA1c and fasting glucose. Your three-month blood-sugar average; creeping values respond to diet, activity, and sleep far more than to any supplement.
• Triglycerides ÷ HDL. A quick insulin-resistance proxy you can compute from any lipid panel: a ratio above ~3 (in mg/dL) suggests metabolic work to do.
• CRP. General inflammation. Also matters for interpretation: inflammation raises ferritin, masking iron deficiency.

Trends beat snapshots

A single test is a photo; two or three over a year are a film. Retesting 8–12 weeks after starting a supplement is the only way to know it's working — vitamin D and ferritin in particular respond slowly and measurably.

Before your next draw

• Fast if the panel includes glucose or triglycerides (8–12 hours, water is fine).
• Skip biotin (B7, common in "hair & nails" formulas) for ~48 hours — it can distort thyroid and hormone assays.
• Test at a consistent time of day; several hormones swing daily.