Haematology Analyser Flags: What They Mean and What to Do
A flag is the analyser saying “I counted this, but I am not sure”. What the common flags point to, which come from the sample rather than the patient, and the checks before a flagged result goes out.
Three kinds of flag
Every maker names and displays its flags differently — some add a question mark, some use codes or coloured markers — but they fall into three groups. Learn the exact list for your model from its instructions for use.
Suspect cell flags
The analyser has seen a population that may contain abnormal cells — blasts, immature granulocytes, atypical lymphocytes or nucleated red cells. These need a blood film.
Sample and interference flags
Something in the tube is distorting the count — platelet clumps, red-cell agglutination, lipaemia, haemolysis or an old sample. These are usually solved at the sample, not the microscope.
Instrument flags
Clogs, bubbles, low reagent or a failed background count. Do not report the result until the fault is cleared and the sample rerun.
Clogs, bubbles and errorsWhite-cell flags
| Flag | What it may mean | What to do |
|---|---|---|
| Abnormal WBC scattergram or histogram | Populations do not separate cleanly — abnormal cells, an old sample, or interference at the low end | Check the sample age, mix and rerun; make a film if it persists |
| Blasts | Possible blast cells. Leukaemia has to be excluded, although other cells can trigger the flag | A film reviewed by trained staff the same day; urgent referral if blasts are confirmed |
| Immature granulocytes (IG) or left shift | Myelocytes, metamyelocytes or band forms — common in bacterial infection, sepsis, pregnancy and after G-CSF; occasionally a marrow disorder | Film; where the analyser reports an IG count, your review rules may set a level below which no film is needed |
| Atypical or abnormal lymphocytes | Reactive lymphocytes in viral infections, including dengue and glandular fever; occasionally lymphoma or chronic leukaemia | Film, read with the clinical picture and the patient’s age |
| NRBC (nucleated red cells) | Some analysers count them as white cells. Normal in newborns; in adults, seen with haemolysis, severe anaemia, thalassaemia major or marrow disease | Use the analyser’s NRBC-corrected WBC, or correct it from a film count |
| Raised mid-cells (MID) on a 3-part | More cells in the middle size band — monocytes, eosinophils, basophils or abnormal cells | A film if the cause matters; a 5-part analyser separates them |
Red-cell and sample flags
These flags usually describe the sample, not the patient. The giveaway is often an index that cannot be real — most often an MCHC far above the reference range.
| Flag or pattern | Usual cause | What to do |
|---|---|---|
| RBC agglutination; very high MCHC with low RBC and high MCV | Cold agglutinins clumping red cells at room temperature | Warm the sample to 37 °C as your procedure states and rerun promptly |
| High HGB, MCH and MCHC with normal-looking red cells | Lipaemia, or turbidity from a very high white-cell count, raising the haemoglobin reading | Follow your lab’s correction procedure, such as plasma replacement, or refer |
| Low RBC and HCT with unchanged HGB; high MCHC | Red cells broken in the tube (haemolysis) after a difficult draw or rough handling | Recollect with better technique |
| Two red-cell populations, high RDW | Recent transfusion, iron deficiency under treatment, or a mixed-up sample | Check the history; make a film if unexplained |
| High MCV and HCT with a poor differential | An old sample — red cells swell and white cells deteriorate on standing | Note the collection time; recollect if beyond validated stability |
Platelet flags
Platelet clumps
The most common reason for a falsely low platelet count. Confirm on a film, check the tail of the smear, and recollect in citrate if needed.
Platelet clumping step by stepGiant platelets
Very large platelets can fall outside the platelet size window of an impedance count and be missed, lowering the count. An optical or fluorescent platelet count, where available, or a film estimate helps.
Small particles counted as platelets
Red-cell fragments and very small red cells — common in severe thalassaemia — can be counted as platelets by impedance, raising the count. The histogram shape and a film show it.
Before a flagged result goes out
- Check the sample first: patient identity, clots, fill line, collection time, lipaemia and haemolysis.
- Mix well and rerun. A flag that disappears on a rerun of a well-mixed sample was often a sampling artefact — but record it.
- Compare with the patient’s previous results. An unexplained change (a failed delta check) is a reason for review in itself.
- Make a film when your written review rules say so. Many laboratories adapt the ISLH consensus criteria to their analyser.
- Report with a comment rather than ignoring the flag, and refer to a laboratory when no trained reviewer is available on site.
- Never switch flags off to speed up reporting. Flag settings belong in the laboratory’s documented procedure.
Read next Review criteria and the ISLH rulesQC and accreditation
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Frequently asked questions
What does a flag on an FBC result mean?
Does a blast flag mean leukaemia?
Why is the MCHC so high?
Do 3-part analysers flag abnormal cells?
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