How to read your blood test results.

The anatomy of a result page.

Most U.S. lab reports — whether from Quest, LabCorp, an at-home provider like Function Health, or a hospital system — follow a similar layout. Once you've seen one, you've roughly seen them all.

At the top: your name, the ordering provider, the date of the draw, and a specimen ID. Then a table that looks something like this:

Below the table you may find footnotes, methodology notes (which assay was used) and sometimes a short interpretation paragraph from the lab. None of that is a diagnosis. The diagnostic conversation belongs with the clinician who ordered the test, in the context of your symptoms and history.

What a reference range actually is.

This is the part that surprises most readers. A reference range is not a clinical target. It is a statistical description — usually the middle 95% of results from people the lab has tested. That means roughly 2.5% of healthy people will sit above it, and 2.5% will sit below it, just by chance.

"The reference range answers a population question. Your provider has to answer an individual one."

Two consequences follow. First, two laboratories can publish different ranges for the same biomarker, because their patient mix and machine calibration differ. Second, a number that is "in range" at the lab cutoff is not automatically the number that's right for you. Ranges are a starting frame, not a verdict.

This matters especially for U.S. women navigating ferritin, TSH, vitamin D, B12 and estradiol, where the population range and the optimal individual range can diverge in clinically meaningful ways.

Population vs. functional ranges.

Some clinicians use the phrase "functional" or "optimal" range to describe a tighter band — the values at which symptoms are least common, drawn from clinical observation and the research base. This is a conversation, not a fixed standard. A value that sits in the population range but outside a functional range may be worth discussing with a qualified healthcare provider, particularly if you have symptoms.

What L, H and other flags mean.

The flag system is simple, and most U.S. labs follow it.

• L (low) — your number is below the lab's lower reference limit.
• H (high) — your number is above the lab's upper reference limit.
• A (abnormal) — used by some labs in place of L/H to flag any value outside range.
• Critical / panic flags — used for values so far from typical that the lab will usually call the ordering provider directly (very high potassium, very low haemoglobin, etc.).

The important thing to internalise: a flag is a screen. It is not a diagnosis, and it is not graded by severity. A ferritin of 14 ng/mL with no flag (because the lab's cutoff is 12) is not "fine" if your hair is shedding and you exercise — and a TSH of 4.6 mIU/L with an H flag is not always an indication for medication. The flag tells you to look; the clinical picture tells you what to do.

Units, decimal places and why they matter.

Two countries, two unit systems. The U.S. uses conventional units — ng/mL, mg/dL, pg/mL — while much of the rest of the world uses SI units (nmol/L, mmol/L). If you've ever compared an American result to a U.K. one and felt like the numbers couldn't possibly be the same biomarker, that's usually why.

The conversions that catch people out.

• Glucose: 100 mg/dL in the U.S. = 5.6 mmol/L in SI units. Divide mg/dL by 18.
• Cholesterol: 200 mg/dL in the U.S. = 5.2 mmol/L. Divide mg/dL by 38.7.
• Vitamin D (25-OH): 30 ng/mL = 75 nmol/L. Multiply ng/mL by 2.5.
• Estradiol: 50 pg/mL = 184 pmol/L. Multiply pg/mL by 3.67.

If you're cross-referencing a clinical guideline written in SI units (most international ones are), make sure your number is in the same system before drawing any conclusions. The unit is not decorative.

The abbreviations worth knowing.

U.S. lab reports lean on shorthand. Here are the ones you're most likely to see — what they stand for, what they actually measure, and where to read more.

A full glossary of abbreviations, with what each unit looks like in U.S. and SI form, sits on the biomarkers index.

When "normal" doesn't mean optimal.

This is the section every reader of women's health writing eventually arrives at. A result can be "in range" and still be worth a closer look — particularly for the markers most affected by menstruation, pregnancy, postpartum and the perimenopause transition.

Where the gap shows up most often.

A few of the patterns U.S. clinicians familiar with women's health describe regularly:

• Ferritin between 15 and 30 ng/mL. Most labs flag below 12 or 15. Many clinicians consider stores below 30 ng/mL inadequate for menstruating women — and below 50 ng/mL inadequate when hair shedding is part of the picture. See the ferritin guide for the full discussion.
• TSH in the 3.0–4.5 mIU/L window. Often flagged only above 4.5. Some clinicians prefer to see TSH closer to 2.5 mIU/L, particularly in women trying to conceive or already pregnant.
• Vitamin D in the low 30s ng/mL. Below 30 is often flagged "insufficient" rather than "deficient." Many practitioners aim for 40–60 ng/mL for general health, with caveats for kidney health and supplementation context.
• B12 in the 200–400 pg/mL band. "Normal" at most labs, but the lower end of this band is increasingly linked to neuropathic and cognitive symptoms in symptomatic patients.
• Fasting insulin above 7–8 µIU/mL. Rarely flagged. Often the earliest signal of metabolic drift, well before glucose or HbA1c move.

None of these are diagnoses. They are conversations — the kind worth having with a qualified healthcare provider who knows your full history.

Bringing the report to your provider.

A blood test result page is most useful in the room with the person who ordered it. A few practical moves make that conversation better.

Track results over time.

A single snapshot can mislead. A trend rarely does. Keep your last two or three sets of results in one place — a notes app or a printed folder — and look at them together. A ferritin moving from 60 to 30 to 18 over three years tells a different story than a single 18.