H and L Flags Explained

Section 1

What H and L flags actually are

H and L flags are generated automatically by the laboratory information system — a computer comparison of your result against the stored reference range.

When your result is returned from the analyser, the laboratory software instantly compares it to the reference range stored in its database. If your result exceeds the upper limit, the software prints "H". If it falls below the lower limit, it prints "L". No human makes this decision.

The system does not know your age (in many cases), your symptoms, your medications, or your previous results. It is a simple greater-than or less-than comparison. This is why flags must always be interpreted by a clinician, not read in isolation.

Computer-generated, not doctor-assessed

An H or L flag appears on every report where the result falls outside the statistical reference range — whether the deviation is 0.1 units or 10 units. The flag does not reflect clinical significance.

Section 2

The flag hierarchy: from borderline to critical

Not all flags carry equal weight. Understanding the hierarchy helps you respond proportionately.

Most results that are flagged fall just outside the reference range — perhaps 5–10% above or below the limits. These borderline deviations are the most common, the least clinically significant, and the most likely to be due to biological variation or a slightly narrow reference range.

H / L

Borderline flag

Result is outside the reference range but not severely so. Most common. Often not clinically significant. Discuss at your next appointment.

H* / L*

Abnormal flag

Result is notably outside range. More likely to require attention. Your doctor will likely contact you if this is new or changing.

HH / LL

Critical value

Result is far enough outside range to potentially require immediate action. Lab policy mandates calling the ordering physician within minutes.

↑↑ / ↓↓

Panic value

Alternative notation for critical values. Same urgency as HH/LL. Examples: potassium <2.5 or >6.5 mmol/L, sodium <120 mmol/L.

Section 3

Flags that are often clinically insignificant

Some tests are routinely flagged in healthy people because reference ranges are drawn from populations that don't account for individual variation.

MCV (mean corpuscular volume) is frequently flagged as mildly high in otherwise healthy individuals, particularly those who drink moderate amounts of alcohol or take certain medications. A mildly elevated MCV without anaemia or B12/folate deficiency rarely requires treatment.

Monocytes, eosinophils, and basophils are often flagged as mildly elevated during minor viral infections or seasonal allergies. These white cell fractions fluctuate with immune activity and are clinically significant only when substantially elevated or persistently rising.

Mild and stable = usually not urgent

If a result has been mildly flagged at the same level across multiple tests over months or years, your doctor has likely already assessed it as your personal baseline. Bring it up at your next scheduled appointment rather than seeking an urgent review.

Section 4

When to call your doctor about a flag

A flag alone does not tell you how urgently to act. These are the situations that warrant prompt contact.

Call your doctor the same day if a critical value (HH/LL) appears on your results, if you have new or worsening symptoms alongside any flag, if a result has changed significantly since your last test, or if you have a known condition that makes certain flags more significant (e.g. kidney disease and potassium, heart failure and sodium).

Do not call after hours for a mildly borderline H or L that has been present on previous tests, or for a single flag in the context of a recently resolved illness. These can be raised at your next scheduled appointment or via a patient portal message.

Section 5

Delta checks — flagging changes, not just absolutes

Beyond H and L flags for absolute values, many laboratories also run delta checks — flags that alert when a result has changed significantly from the previous one.

A delta check compares your current result to your most recent result at the same laboratory. If the change exceeds a defined threshold, it is flagged for pathologist review — even if both results are within the reference range.

Delta flags serve dual roles: quality control (catching specimen mix-ups or errors) and clinical alerts (flagging rapid physiological change). A sodium that drops 8 mmol/L in 24 hours flags regardless of whether both values are technically "in range".

Delta checks catch errors as well as changes

If your result is dramatically different from your last one and you feel well, a sample or labelling error is possible. Your doctor may order a repeat before acting on a large unexpected change.

Section 6

Non-disease causes of flagged results

Many commonly flagged results have explanations unrelated to disease. Understanding these prevents unnecessary anxiety.

Haemolysis — the breaking of red cells after blood draw — falsely elevates potassium, LDH, AST, and phosphate. A haemolysed sample drawn with difficulty or transported incorrectly can produce alarming-looking flags that disappear on a clean repeat draw.

Lipaemia (excess fat in non-fasted blood) interferes with many colorimetric assays. Triglycerides, albumin, and some enzyme assays can all be falsely elevated or suppressed in a lipaemic sample — which is why fasting is required for lipid panels.

Section 7

Reading flag patterns, not isolated flags

A single flagged result is often uninformative. A pattern of related flags across a report tells a much more coherent story.

Low haemoglobin, low MCV, and low MCH flagged together strongly point to iron deficiency or thalassaemia trait. High calcium and high alkaline phosphatase together point to bone pathology. The pattern is the finding.

When reviewing a flagged report, list everything that is flagged and ask whether the flags cluster around a single system or organ. If they do, there is likely a single explanation. Scattered, unrelated flags across multiple systems are more often due to laboratory or pre-analytical factors.

Section 8

A practical framework for responding to flags

When you receive a report with flags, use this four-step sequence before deciding how urgently to respond.

1

Check for critical values (HH / LL)

If any result is marked HH, LL, or critical, your doctor should already have been called. If they have not contacted you, call them the same day.

2

Check magnitude

Is the flagged value 5% outside the range or 50% outside? Borderline flags in otherwise well patients are rarely emergencies.

3

Compare to your previous results

Is this flag new, or has it been present across several tests? A chronic stable flag is far less urgent than a new one.

4

Match the flag to your symptoms

A flag that aligns with symptoms you are experiencing deserves prompt attention. A flag in someone who feels entirely well is far less likely to require urgent action.