- 01A flag is statistics, not a verdict
H and L mean your number fell outside the middle 95% of a healthy reference group. One in 20 healthy results does that by design, so on a 20-test report most healthy people see at least one flag.
- 02Critical flags are a different category
HH, LL, a star or the word critical means the result crossed a limit the lab treats as dangerous, and US labs must alert the ordering clinician immediately. That flag needs a same-day conversation.
- 03Some words describe the sample, not you
Hemolyzed, lipemic, QNS and specimen rejected describe the tube. A hemolyzed sample can show a falsely high potassium, and QNS means there is no result at all.
An H on a blood test means the result sits above the reference range printed next to it, and an L means it sits below. That is all a plain H or L says: the number fell outside the band where the middle 95% of a healthy reference group landed. The letter does not say how far outside, why, or whether it matters for you. A small set of other marks does carry urgency, and those are the ones worth learning to spot.
Every flag on a report is a comparison. The lab takes your number, holds it against the reference range it uses for your age and sex, and prints a letter when the number falls outside. The letter carries no medical judgment. A potassium of 5.2 mmol/L against a range ending at 5.1 gets the same H as a potassium of 6.8 mmol/L, and those two results mean very different things.
What do H, L and the other flags mean on a lab report?
| Flag | What it means | How urgent it usually is |
|---|---|---|
| H | Above the reference range | Usually routine. Read how far above, and the notes. |
| L | Below the reference range | Usually routine. Read how far below, and the notes. |
| HH, LL, H*, L*, CH, CL, or the word critical | Far enough outside the range to cross the lab's critical limit | Same day. The lab must alert the ordering clinician. |
| A | Abnormal, used for results that are not a number (positive, reactive, detected) | Depends on the test. Read the comment. |
| AA | Critically abnormal non-numeric result | Same day. |
| > or < before the number | The result is beyond what the instrument can measure (for example, >500) | Depends on the test and the direction. |
| Blank or N | Inside the reference range | No flag. Inside the range is not the same as ideal. |
These letters follow a common electronic messaging convention that most US labs and patient portals use, but the exact symbols vary. Some portals show colours or arrows instead of letters, and some print H* for a critical high. The key at the foot of your own report is the final word on what each mark means at that lab.
What is a reference range on a blood test?
A reference range (also called a reference interval) is the band of results seen in a group of healthy reference people. By convention the lower limit is the 2.5th percentile and the upper limit is the 97.5th percentile, so the range holds the central 95% of that group, and the standard method calls for at least 120 reference people to set the limits with confidence. 1 Each lab sets or checks its own range for its own instruments, and US federal rules require the lab to make those ranges available to the clinician who ordered the test. 4 That is why the same potassium can be in range at one lab and flagged at another, and why a result should be read against the range printed on its own report. The guide to reference ranges covers why ranges differ and why in range is not the same as optimal, and the guide to lab units covers mg/dL against mmol/L.
Why do healthy people get abnormal blood test results?
If each test has a 5% chance of flagging a healthy person, a report with many tests has a much higher chance of flagging at least one. With 20 independent tests, the chance that every one lands inside its range is 0.95 multiplied by itself 20 times, about 36%. That leaves about a 64% chance of at least one H or L in a healthy person. A comprehensive metabolic panel has 14 lines and a complete blood count with differential has 20 or more, so a routine set of labs easily passes 20 results.
The arithmetic cuts both ways. A flag in a healthy person is common, and a result inside the range does not prove health either, because the range describes a population and not one person. The guide on how to read any lab result covers that second half.
What a mild flag often reflects
- Normal day-to-day variation. Many markers move several percent between draws in the same healthy person.
- A recent meal. Glucose and triglycerides rise after eating, which is why some tests are drawn fasting.
- Hard exercise in the day or two before the draw, which can lift muscle enzymes such as AST and creatine kinase.
- Hydration. A dehydrated draw concentrates hemoglobin, albumin and BUN; a lot of fluid dilutes them.
- Time of day, for markers with a daily rhythm such as cortisol and testosterone.
- How the sample was handled: a long tourniquet, a delay before processing, or burst red cells in the tube.
- A reference group that does not match you, for example a range not adjusted for age, sex or pregnancy.
What are critical lab values?
A critical value (older reports say panic value) is a result far enough from normal that it can threaten life if nobody acts soon. George Lundberg, a pathologist whose hospital team set up one of the first critical value systems in the early 1970s, later set out the definition that labs still use:
A critical (or panic) laboratory value is a laboratory test result that represents a pathophysiologic state at such variance with normal as to be life-threatening if an action is not taken quickly and for which an effective action is possible.Lundberg GD. MedGenMed, 2007 2
Under the federal rules that govern US labs, the lab must immediately alert the person or office that ordered the test when a result points to an imminently life-threatening condition or crosses a panic or alert value. 4 In practice that is usually a phone call, often with the receiver reading the result back to confirm it. There is no single national list of critical limits: each lab writes its own, and the limits differ from lab to lab. 3 The examples below come from a 2012 survey of 36 laboratories in Australia and New Zealand. They show how far apart those limits can sit for three common tests; they are not US standards, and your own lab's limits may differ.
| Test (examples from one 2012 survey) | Critical low (median, range across labs) | Critical high (median, range across labs) |
|---|---|---|
| Potassium | 2.8 mmol/L (2.2 to 3.0) | 6.0 mmol/L (5.4 to 6.9) |
| Sodium | 125 mmol/L (120 to 130) | 155 mmol/L (150 to 160) |
| Glucose | 2.5 mmol/L, about 45 mg/dL (1.5 to 3.0 mmol/L) | 20 mmol/L, about 360 mg/dL (8 to 30 mmol/L) |
Those figures come from Campbell and Horvath's 2012 survey of Australasian labs and are examples only; critical limits vary by lab. 3 Many labs also add hemoglobin, platelet count, white cell count, calcium, INR and positive blood cultures to their own lists, each with limits the lab sets.
Many results now reach patient portals as soon as the lab releases them, often before the ordering clinician has looked. That is why people meet flags on their own, at night, with nobody to ask. Most of those flags are mild, and the clinician's office will often have already seen the critical ones because the lab had to phone them first.
What do hemolyzed, QNS and see note mean?
Reports also carry words about the sample, the method or the result itself. These describe what happened in the tube or in the lab, not something happening in your body.
| Term on the report | What it means |
|---|---|
| Reference range, reference interval, normal range | The band holding the middle 95% of the lab's healthy reference group. |
| See note, see comment, footnote | The lab added a remark. It may explain the method, warn that the sample was not ideal, or give an interpretation. Read it before reading the number. |
| Hemolyzed, hemolysis, H index | Red cells burst in the sample, usually during the draw or handling. Some results read falsely high or low. |
| Lipemic | The sample was cloudy with fat, often after a meal. Some methods read less accurately. |
| Icteric | The sample was yellow from bilirubin. Some methods read less accurately. |
| QNS (quantity not sufficient) | There was not enough sample to run the test. There is no result, and a redraw is usually needed. |
| Specimen rejected, test not performed, TNP | The sample failed the lab's acceptance checks (clotted, wrong tube, unlabelled, too old). No result. |
| Pending, in process | The result is not ready yet. Some tests take days. |
| Corrected report, amended | The lab changed an earlier result. The law requires the lab to notify the ordering clinician and keep both versions. 4 |
| Verified on repeat, confirmed | The lab ran the sample again and got the same answer. Labs often do this for surprising or critical results. |
| Calculated | The value is worked out from other results (eGFR, LDL in some labs, globulin, anion gap) rather than measured. |
Why a hemolyzed sample can show a high potassium
Red blood cells hold far more potassium, LDH and AST than the fluid around them. When cells burst in the tube, those contents spill into the serum, and the lab measures a potassium, LDH or AST that is higher than the level in your bloodstream. Lippi and colleagues found clinically meaningful shifts in potassium, LDH and AST even when hemolysis was barely visible. 5 A high potassium on a sample marked hemolyzed is usually redrawn before anyone reads anything into it. A high potassium on a clean sample is read as real.
QNS and specimen rejected are not results at all. They mean the lab could not give a number, and the gap on the report is a missing value rather than a normal one.
What a flag does and does not tell you
| A flag tells you | A flag does not tell you |
|---|---|
| Your number fell outside the lab's reference range | Whether you have a disease |
| Which direction it fell (above or below) | How far outside it is; read the number against the limit |
| For a critical flag, that the lab judged it dangerous enough to phone about | Why the number is where it is |
| That this line deserves a closer look | Whether it has changed from your previous results |
Distance from the limit matters more than the letter. A result 2% past the edge of a range is a different finding from one at twice the upper limit. Direction matters for each test in its own way: a low lymphocyte percentage often flags only because another cell type rose, while the absolute lymphocyte count on the same report sits in range. The CBC guide explains why the absolute counts are the ones to read.
How to read a flagged result, step by step
- 1Check for a critical mark firstIf any line is marked critical, HH, LL, AA or with a star, that line needs a same-day conversation with the ordering clinician. Everything else can wait for the steps below.
- 2Read the range and units on your own reportCompare the number with the limits printed beside it, in the same units. Work out roughly how far past the limit it sits.
- 3Read the notesA comment such as hemolyzed, non-fasting or lipemic can explain a flag before you look any further.
- 4Read the neighboursTests are built to be read together. A high BUN with a normal creatinine tells a different story from both rising together. A low hemoglobin reads differently depending on the red cell size beside it.
- 5Compare with your previous resultsA value that has sat slightly above the range for five years is a different finding from one that jumped this month. The guide to trends explains why the direction of travel often says more than one reading.
- 6Ask the clinician what they make of itUseful questions: is this far enough outside to matter, does it fit with my other results and symptoms, and would a repeat test help.
When a result does not fit the person
Sometimes the number is wrong in a way no flag can show. Biotin, a common hair, skin and nail supplement, can distort tests that use a biotin and streptavidin step, and the FDA has received reports of falsely low troponin results in people taking it, a reading that could make a heart attack look like no heart attack. 6 Clinicians are trained to question a result that does not match the person in front of them. If you take biotin or any high-dose supplement, the lab and the clinician reading your results should know. The biotin guide covers which tests it affects.
Which flags usually wait for a routine conversation?
A single H or L, close to the limit, with no critical mark, no symptoms and no worrying trend, is usually discussed at the next routine contact, and the most common plan is a repeat test. Many such flags settle on the second draw. A result well outside its range but short of critical, or one that is new and moving in the wrong direction, is worth raising with the ordering clinician within days rather than at next year's check-up.
Link · MedlinePlus, US National Library of MedicineHow to understand your lab resultsA plain-language reference on reference ranges, false positives and false negatives, and the questions to ask about a result.medlineplus.govThe clearest way to read any flagged line is in this order: is it critical, how far outside the range is it, what do the notes and neighbouring results say, and how does it compare with last time. Most H and L flags on a routine report end at the second or third question.
Citations
- Ozarda Y. Reference intervals: current status, recent developments and future considerations. Biochem Med (Zagreb). 2016;26(1):5-11.
- Lundberg GD. It is time to extend the laboratory critical (panic) value system to include vital values. MedGenMed. 2007;9(1):20.
- Campbell CA, Horvath AR. Towards harmonisation of critical laboratory result management: review of the literature and survey of Australasian practices. Clin Biochem Rev. 2012;33(4):149-160.
- 42 CFR 493.1291, Standard: Test report (Clinical Laboratory Improvement Amendments regulations), including paragraphs (d) reference intervals, (g) immediate alert for imminently life-threatening results, and (k) corrected reports.
- Lippi G, Salvagno GL, Montagnana M, Brocco G, Guidi GC. Influence of hemolysis on routine clinical chemistry testing. Clin Chem Lab Med. 2006;44(3):311-316.
- US Food and Drug Administration. Biotin interference with troponin lab tests: assays subject to biotin interference.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
