What Flagged Values on Your Lab Report Mean
A flagged value is a lab result marked H (high), L (low) or another code. It means your number falls outside the lab's reference range. A flag is not a diagnosis. Ranges are built so that about 1 in 20 healthy people land outside them on any single test.
The short version
- H means high and L means low: the result is above or below the lab's reference range. N, or an empty flag column, means normal. A means abnormal for results that aren't numbers.
- If you're healthy and have 20 tests, there's about a 64% chance at least one comes back flagged, a 2025 medical review calculates.
- Critical flags (HH, LL, AA, or a result labeled critical) are different: federal rules require the lab to alert your doctor immediately. If you see one and haven't heard from the office, call the same day.
- For other flags, ask yourself four things. How far out is the number? Is it new? Does it fit the rest of the report? Do you have symptoms?
You probably see your lab results in a patient portal now, often before your doctor does. You open the report, scan the numbers, and a few are marked in red.
Those marks are flags. The lab's software adds them automatically when a number crosses the edge of its range. It knows nothing about you: not your medicines, not your usual numbers, not yesterday's workout.
What do the columns on a lab report mean?
Nearly every lab report is the same table with five columns: test name, result, flag, units and reference range. Only the labels change from lab to lab, which is why a new lab's report can look unfamiliar.
- 1 Test: What was measured
- 2 Result: Your number
- 3 Flag: H = above the range, L = below
- 4 Units: Read these before the number
- 5 Reference range: This lab's range for healthy people
| Column | Also labeled | What it tells you |
|---|---|---|
| Test name | Test, Component, Analyte | Which marker was measured (abbreviations decoded) |
| Result | Value, Your Value | Your number for that marker |
| Flag | Abnormal Flag, In Range / Out of Range | Whether the number falls outside the range |
| Units | Unit, UOM | How the result is measured, such as mg/dL or mmol/L (lab units explained) |
| Reference range | Reference Interval, Ref Range, Ref Value, RefIntvl, Standard Range | The span of results the lab considers typical |
On a Quest Diagnostics report, each result sits in an "In Range" or "Out of Range" column. Numbers outside get an H or L. Patient portals such as MyChart usually spell the flag out (High, Low, Abnormal) and show the range as a bar.
Read the units before the number. A result of 5.4 means something completely different in mmol/L than in mg/dL.
What do H, L, N and A mean on lab results?
H means your result is above the lab's range, L means below it, and N or a blank means inside it. Most labs use the same basic letters, so these codes carry over from one report to the next.
| Flag | Stands for | What it tells you |
|---|---|---|
| H | High | Your result is above the top of the lab's range |
| L | Low | Your result is below the bottom of the range |
| N or blank | Normal | Inside the range; many reports leave the column empty instead of printing N |
| A | Abnormal | Used for results that aren't numbers, such as a positive test where negative is expected |
| POS, NEG or ND | Positive, negative, not detected | For yes-or-no tests; on many of them, negative or not detected is the answer you want |
| HH, LL, AA | Critical | Past the lab's alert limit; some reports and portals spell it out as critical |
| < or > | Off the scale | Below or above what the instrument can measure; "<0.5" means too little to measure |
| * (asterisk) or ! | Look here | A general marker for out-of-range or alert results; the report's legend says which |
See an icon that isn't in this table? Check the legend, usually at the bottom of the page. Labs are free to add their own symbols.
Why do healthy people get flagged?
Because a reference range covers only the middle 95% of healthy results. It's meant to be built from at least 120 healthy people, and the highest and lowest 2.5% fall outside it by design. So about 1 in 20 healthy people land outside the range on any single test.
MedlinePlus, the US government's health library, agrees: it's common for healthy people to have some results outside the range. Ranges also change with age, sex and pregnancy. That's one reason the reference range on your report may not match a friend's.
What can push a result out of range?
Often it's the day of the draw, not your health. Timing alone can do it. Some tests, like cortisol, follow a daily rhythm, so the best time for blood work depends on the test.
Other common culprits, according to MedlinePlus:
- What you ate or drank beforehand, or skipping a fast the test required (the fasting rules differ by test)
- Medicines and supplements you take
- A hard workout before the draw
- Having your period at the time of the test
- How the blood was drawn: clenching and relaxing your fist during the draw can temporarily push potassium up
Why is a cholesterol flag different?
Because its line marks where heart risk starts to rise, not where most healthy people fall. Plenty of healthy adults sit above it.
So a flag on your lipid panel doesn't mean your number is unusual. It means you're past the level linked with heart risk, which is worth discussing even if you feel fine. A few other tests work the same way, with a line set by health risk rather than by healthy averages.
What does a critical flag mean?
A critical flag (HH, LL, AA, or a result labeled critical) is different. Your result crossed a second line, much further out. The lab treats it as a possible emergency. Federal rules require the lab to tell whoever ordered the test right away.
The guide to critical lab values covers which tests carry these thresholds.
Which flags can wait?
The authors of a 2025 medical review propose that labs label every result with one of three plain tiers:
- No immediate concern
- Discuss with your doctor at the next visit
- Contact your doctor soon
Most reports don't do that yet. Four questions help you judge how soon to bring a flag up:
- How far outside the range is it? A result a hair past the line is a different situation from one far beyond it.
- Is it new? Compare it with your earlier results, ideally from the same lab. Results from one lab vary much less than results across labs (comparing lab results explains why).
- Does it fit the rest of the report? One stray value among normal neighbors means less than several related markers moving the same way.
- Do you have symptoms? A flag plus new symptoms is worth a call even when the number looks mild.
Questions to bring to your appointment
- Is this result far enough outside the range to matter for me?
- Could something on the day of the test explain it, like a workout, a missed fast or a new supplement?
- Is this range based on healthy people, or is it a risk target?
- How does it compare with my previous results?
- Should we repeat the test, and at the same lab?
Your report flags it. We explain it.
Labs mark out-of-range numbers and leave the meaning to you. Upload the report and every flagged value is explained in plain words, right next to the lab's own reference range, so you know what to ask your doctor.
Potassium is slightly above this lab's range
5.3 sits just past the 5.1 upper limit. Several things, including how the blood was drawn, can nudge potassium up, so ask your doctor whether a recheck makes sense.
Quick answers about lab flags
What does flag N mean on lab results?
What does H or L mean on a blood test?
What does HH or LL mean on lab results?
What does RefIntvl mean on a lab report?
What does a < or > sign mean next to a result?
Should I worry if my blood test is slightly out of range?
Sources
- MedlinePlus: How to Understand Your Lab Results
- MedlinePlus: Potassium Blood Test
- Doles N, et al. Interpretating Normal Values and Reference Ranges for Laboratory Tests. J Am Board Fam Med, 2025
- HL7 FHIR R4: Table 0078, Abnormal Flags (Interpretation Codes)
- 42 CFR § 493.1291, Standard: Test report (CLIA)
Written by BloodSight Editorial Team · Last updated
Disclaimer
This content is for informational and educational purposes only. It is not intended as medical advice, diagnosis, or treatment recommendation. Reference ranges may vary by laboratory. Always discuss your results with a qualified healthcare professional.
More Guides
Blood Test Results Explained: A Plain-Language Guide
Blood test results are the values a lab measured in your sample, each shown next to that lab's reference range. Reading them well means going panel by panel, checking the flags, and comparing each number with its range and your earlier results.
What Is a Reference Range?
A reference range, also called a reference interval or normal range, is the span of results a lab expects from healthy people. It's usually the middle 95% of a healthy group. Think of it as a yardstick, not a verdict. Some healthy people land outside it, and a result inside it can still be worth a closer look.
What Are Critical Lab Values?
A critical lab value, also called a panic value, is a result so far outside the usual range that it may signal a life-threatening problem. The lab must report it to your doctor right away. Each lab sets its own limits: Labcorp alerts on potassium below 2.5 mmol/L, ARUP below 3.0.
How to Track Lab Results Over Time
Tracking lab results means comparing the same test across dates to see if it's stable, rising or falling. The tricky part is telling a real change from normal ups and downs. For total cholesterol, two results need to differ by more than about 17.5% before the change is likely real.
Why Lab Results Differ Between Labs (and How to Compare Them)
Your blood can give slightly different numbers at different labs. Each lab uses its own equipment, test methods and normal ranges. A few tests, like cholesterol, creatinine and HbA1c, are calibrated to one shared worldwide standard and compare better. For most others, compare each result with its own lab's range.