The First Steps With Blood Tests Explained
Blood Tests Explained: the few things that matter most, and the mistakes that catch people out early.
What the common blood tests measure and how to read the results sensibly.
Key takeaways
- Reference range: defined so that some healthy people fall outside it
- Trend: more informative than a single value
- Most useful single change: Ask for the actual numbers and the reference range.
- Commonest mistake: Reading a slightly abnormal value as a diagnosis.
- See a doctor if any of the signs under "When to speak to a doctor" below apply to you.
Why bother
Reference ranges are statistical, so a proportion of healthy people fall outside them by definition.
Start here
These two first. Everything else is easier once they are habit.
- Ask for the actual numbers and the reference range
- Ask what would change your care before requesting more tests
Then add these
Once the basics are in place, add these.
- Have borderline results repeated rather than escalated immediately
- Keep your own record or use your patient portal so trends are visible
The numbers, in one place
| Measure | Figure |
|---|---|
| Reference range | defined so that some healthy people fall outside it |
| Trend | more informative than a single value |
| CBC | complete blood count |
| BMP | basic metabolic panel, including kidney function and electrolytes |
| CRP | C-reactive protein, a marker of inflammation |
Common early mistakes
- Reading a slightly abnormal value as a diagnosis
- Ordering broad direct-to-consumer panels without a clinical question
- Acting on results taken during an acute illness
Related topics
- Having a Blood Test: Sorting the Evidence From the Noise (Using Health Services)
- What the Guidance Really Says About HbA1c (Diabetes)
- What the Guidance Really Says About Thyroid Function Tests (Thyroid and Hormones)
- What the Guidance Really Says About Cardiovascular Risk (Heart Health)
Where to go next
- Blood Tests Explained, Explained Without the Jargon
- The Blood Tests Explained Checklist: What to Do and What to Skip
- Blood Tests Explained — the reference entry
- Blood Health: The Complete Guide
- The Evidence on Anemia, Without the Hype
- Vitamin B12 Deficiency: Sorting the Evidence From the Noise
When to speak to a doctor
Contact your doctor promptly if a result is flagged as significantly abnormal, or if you develop symptoms while waiting for results. Seek urgent care for fever with a known low white cell count, particularly during chemotherapy, or for unexplained bruising or bleeding. Call 911 for chest pain, severe shortness of breath or fainting.
Where this comes from
- MedlinePlus — How to Understand Your Lab Results
- National Heart, Lung, and Blood Institute — Blood Tests
How this page was put together
Every figure, recommendation and warning above is taken from the guidance named in Where this comes from. Nothing on this page is inferred, summarized or generalized beyond what those bodies publish: where they give a number, it is their number, and where they are silent, so are we. Pages are rebuilt whenever the underlying record changes. Sign-off by a named clinician, where it has happened, is shown at the top of the page along with the date.
Medical disclaimer
This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to your doctor, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 911.
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