Your lab report, explained — every section, every symbol.
You've received a lab report and it looks like a wall of numbers, abbreviations, and flags. This guide walks you through every section, in order, so you understand exactly what you're looking at before your next doctor's appointment.
The anatomy of a lab report
Most lab reports share the same layout regardless of which laboratory produced them. Understanding this structure once means you can read any report.
At the top you'll find patient information — your name, date of birth, the ordering physician, and the date the specimen was collected. Always check these match your details before reading anything else.
Below that sits the main results table. Each row is one test. You'll see columns for the test name, your result, the reference range (what is considered normal for the general population), the unit of measurement, and sometimes a flag column.
Reading the reference range column
The reference range is the single most important column for interpreting your results — and the most misunderstood.
A reference range shows the values seen in approximately 95% of healthy people. This means that 5% of completely healthy individuals will fall outside the range on any given test.
Reference ranges are calculated from large population studies. They vary by laboratory, by age, by sex, and by the specific method the lab uses to run the test. This is why a result that is "out of range" at one lab may be normal at another.
H, L, and other flag symbols
Flag symbols are automatic alerts generated by the laboratory's computer system — they do not mean your result is dangerous.
H means your result is above the upper limit of the reference range. L means it is below the lower limit. Some labs use ↑ and ↓ or "HIGH" and "LOW" instead.
Critical flags — sometimes shown as HH, LL, or marked in red — indicate a result so far outside the normal range that the lab is required to call your doctor immediately. These are rare.
Units of measurement
Every lab result has a unit — the measurement scale used to express it. Misreading units is a common source of unnecessary worry.
Common units include mg/dL (milligrams per decilitre), mmol/L (millimoles per litre), g/L (grams per litre), and ×10⁹/L (billions of cells per litre). The unit always sits alongside both your result and the reference range.
The same test may be reported in different units by different labs — particularly between countries. UK labs often use SI units (mmol/L) while US labs use conventional units (mg/dL). Always compare your result to the reference range on the same report, not ranges from the internet.
Comparing results over time
A single result tells you where you are today. A series of results tells you whether you're moving in the right direction.
Most labs provide a delta check — a comparison of your current result against your last result from the same laboratory. This appears as a trend column, a percentage change, or a note.
When comparing results across different labs or time periods, look at the trend rather than fixating on whether a specific number is in or out of range. A result that has been stable at 5.8 mmol/L for three years carries very different meaning to one that has risen from 4.2 to 5.8 in six months.
What to do when a result is flagged abnormal
Receiving a flagged result in an online patient portal before speaking to your doctor is increasingly common — and a source of significant anxiety. Here is how to approach it calmly.
First, note the magnitude. A result that is 2% above the upper reference limit is very different from one that is 200% above it. The flag itself does not tell you severity — you need to look at the actual numbers.
Second, note whether this is new or persistent. Log in to previous reports if available. A result that has been mildly elevated for years is almost certainly known to your doctor and assessed as stable. A value that has recently changed significantly is more likely to need discussion.
Reading results through a patient portal
Online patient portals now deliver lab results directly to patients — often before a clinician has reviewed them. This creates specific challenges.
Most portals use colour coding (red for out-of-range) and bold flags that are visually alarming even when the clinical significance is low. The design prioritises visibility, not clinical context. A mildly elevated cholesterol may appear in large red text even if your doctor considers it insignificant.
If results appear in your portal before you have spoken to your doctor, avoid searching each abnormal result individually online. Out-of-context searching reliably leads to worst-case interpretations. Instead, write down your specific questions and bring them to your appointment or send a message through the portal.
Common questions about reading lab reports
The most frequently asked questions from patients receiving their first lab reports.