Reading your own lab report

What you are holding

A laboratory report is a list of measurements with a reference interval beside each one and, usually, a flag where a value falls outside it. It is a measurement document, not a clinical opinion, and it was not written to be read by you.

Understanding its structure makes it far less alarming.

The reference range is not a health range

This is the single most misunderstood thing on the page. A reference interval is usually constructed to contain the central 95% of results from a reference population.

Two consequences follow directly:

• About one in twenty healthy people falls outside it on any given test, by construction. On a twenty‑line panel, a flagged value in a healthy person is close to expected.
• "Within range" is not the same as "good". The reference population was not necessarily healthy — the argument over ALT ranges is exactly this, since the populations used contained many people with undiagnosed fatty liver.

So a flag is a prompt to look, not a diagnosis. And an unflagged value is not a certificate.

Why ranges differ between laboratories

Different analysers, different assay chemistries, different reference populations. A result of 4.8 can be normal at one laboratory and flagged at another, with nothing having changed in you.

This is why a result must always be read against its own report's range, never against a range from the internet or from a previous laboratory — and why comparing two reports from different laboratories is unreliable.

Units, and the trap for anyone who has moved countries

Costa Rican laboratories generally report glucose and lipids in mg/dL. Europe, Canada and much of the Commonwealth use mmol/L. The same value looks wildly different.

Glucose: divide mg/dL by 18 for mmol/L. Cholesterol: divide by 38.67. If a number looks impossible, check the units before anything else.

How to read it without frightening yourself

1. Check the basics first. Your name, the date, and whether you were fasting. Reports do get mixed up.
2. Read it in groups, not line by line. Glucose and HbA1c together. The lipid panel as a whole. The liver enzymes together. The full blood count as a picture. Single values mean little; patterns mean a lot.
3. Note how far outside the range a flagged value sits. A fraction over the limit is a different thing from double it.
4. Compare with your last report from the same laboratory, if you have one. Direction over time is worth more than any single value.
5. Write down your questions before the consultation, while you are looking at it.

The flags that warrant prompt contact rather than waiting

Most abnormal results can wait for the next appointment. A few should not, and a good laboratory will usually telephone about them:

• A very high or very low potassium or sodium
• Haemoglobin markedly low
• White cell count very high or very low
• Platelets very low
• Glucose very high, particularly with thirst, passing urine frequently and weight loss
• Liver enzymes several times the upper limit

If nobody has contacted you and something on that list is substantially out, telephone rather than waiting.

Keep your own copies

Private results do not appear in the CCSS's EDUS record, and CCSS results do not appear in a private clinic's file. Nobody holds a complete picture of you except you.

Ask for the PDF rather than portal access, keep them in one folder with dates in the filenames, and bring them to appointments. It is unglamorous and it is worth more than any single test.

What a report cannot do

It cannot diagnose. It cannot weigh one abnormal value against your history, your medicines and your symptoms. And it cannot tell you which of twenty numbers actually matters — which is the clinician's job, and the part you are paying for.

The boundary

Reading your own results well means arriving at the consultation with better questions, not arriving with conclusions. The ability to ask "why is this flagged and does it matter" is the useful outcome of this page.