RBC Morphology
A written description of how your red blood cells look under a microscope: their size, color, shape and any unusual features.
AI-verified against cited sources. Awaiting expert reviewAlso called RBC morphLOINC 6742-1
What it is
Under a microscope, red blood cells can differ in size, color and shape. A lab professional, sometimes helped by a digital imaging system, looks at a stained drop of your blood spread on a glass slide. They write down what they see in words, such as 'normocytic, normochromic' or 'mild anisocytosis'. Many labs add a grade, such as 1+ to 3+, to show how many cells have a finding. This comment is usually added when your blood count has a flag or when your clinician asked for a smear review.
Why it matters
A blood count gives averages, such as MCV, the average cell size. The smear shows the cells themselves. It can show cells that are mixed in size, paler than usual, shaped differently, or carrying small inclusions. These details can help your clinician understand a result such as low hemoglobin, a high RDW or an unusual MCV. A blood smear alone usually can't explain a condition, and many findings, especially mild ones, are common. What they mean depends on your other results, your symptoms and your history.
What can affect your result
- How long the blood sat in the tube before the smear was made
- How the smear was spread and stained
- Recent blood transfusion, which mixes donor cells with your own
- Severe illness or stress, which can change how cells look
- The words and grades each lab uses, which differ from lab to lab
- Iron, vitamin B12 and folate levels, which can change red cell size and color
- Blood that sits in the tube for many hours before the smear is made can change how cells look. Labs set their own time limits. One hospital lab, for example, asks that a pathologist smear review be requested within 8 hours of the CBC draw.
- A recent blood transfusion adds donor cells, and severe illness or stress can change how blood cells look.
- Different labs use different words and grading scales, so the same cells may be described differently. International recommendations try to make the wording more consistent.
Any time. The comment describes the blood sample from that draw. The comment describes the cells in the sample drawn that day. Red cells live for roughly 3 to 4 months, so changes in size or shape usually build up and fade over weeks to months.
When to talk to a clinician
Many findings on a blood smear are mild. A few, such as blood parasites or a large number of fragmented cells (schistocytes), can need prompt follow-up, and some labs phone the ordering clinician right away. If your lab flagged the result, or you have unusual findings along with tiredness, shortness of breath, pale or yellow skin, easy bruising or bleeding, fever or unexplained weight loss, contact the clinician who ordered the test soon. Call 911 or your local emergency number for chest pain, severe shortness of breath, fainting or confusion.
Questions you could ask
- Which findings on my smear are worth following up, and which are mild?
- How do the smear findings fit with my hemoglobin, MCV and RDW?
- Should we repeat the blood count or smear, or add tests such as iron, B12 or folate?
Questions people ask
What does 'normocytic, normochromic' mean?
These words describe red cells of usual size (normocytic) and usual color (normochromic). They mean the cells looked typical for size and color on the smear. They don't describe everything in your blood, so your clinician still reads them with your other results.
What do grades like 1+ or 2+ mean?
Labs often grade how many cells show a finding, using a scale such as 1+ to 3+ or 4+, or words such as few, moderate and many. The cutoffs depend on the lab and on the finding, so a 1+ at one lab may not match a 1+ at another. International recommendations try to make grading more consistent. Your lab can tell you its own scale.
Why was a smear review done on my blood?
Labs often review a smear when the analyzer shows a flag, a result changes sharply from your last test, or your clinician asked for it. Any of these can be routine, so the review by itself doesn't say what the smear will show.
Show the science and sources
For clinicians and researchers
RBC morphology is a manual or digital-image review of a stained peripheral blood film, reported as free text (LOINC 6742-1, with separate size 18226-1 and shape 18225-3 codes). The International Council for Standardization in Haematology (ICSH) published consensus recommendations on nomenclature and grading of abnormal red cells, white cells and platelets by manual microscopy. For some red cell abnormalities the group suggested that analyzer parameters may be more accurate and less subjective than microscopic or image-analysis grading (ICSH 2015). Smear review is typically triggered by analyzer flags (suspect or numerical flags) or delta checks, and review criteria are set by each lab (Peripheral Smear Review Working Group, 2026). Plain-language definitions used here: anisocytosis is variation in cell size, which RDW measures numerically; poikilocytosis is variation in shape; hypochromia is reduced color (low MCH or MCHC); polychromasia is bluish-gray larger cells, which correspond to reticulocytes. Schistocytes are 1% or less in healthy adults, and more than 1% on the blood film is considered suspicious for thrombotic microangiopathy, while the analyzer fragmented red cell count can help exclude them (ICSH 2021 update). Classic associations that are not diagnostic by themselves: hypochromic, microcytic cells with iron deficiency (StatPearls), and macro-ovalocytes, hypersegmented neutrophils, Howell-Jolly bodies, Cabot rings and punctate basophilia with a low reticulocyte count in vitamin B12 deficiency (Parakh and Dewan 2026). MedlinePlus lists anemia, sickle cell disease, hemolytic anemia, thalassemia and bone marrow disorders among the conditions that can produce red cell findings. Some labs treat blood parasites on a smear as an urgent result (Labcorp critical values list). Result wording and grading (rare, few, moderate, many, or 1+ to 4+) vary between labs, so unrecognized phrases are treated as free text.
LOINC 6742-1 (primary), 58408-6, 18226-1, 18225-3. Specimen: whole-blood.
Sources
- ICSH recommendations for the standardization of nomenclature and grading of peripheral blood cell morphological features, International Journal of Laboratory Hematology (2015)
- 2021 update of the 2012 ICSH Recommendations for identification, diagnostic value, and quantitation of schistocytes, International Journal of Laboratory Hematology (2021)
- Practical Considerations in the Implementation of Peripheral Smear Review in the Clinical Laboratory, International Journal of Laboratory Hematology (2026)
- Blood Smear, MedlinePlus (NIH National Library of Medicine)
- Peripheral Smear Review by Pathologist (test code PB Sm Path), CRMC Laboratory Test Catalog
- Revisiting the hematological manifestations of vitamin B12 deficiency, Frontiers in Nutrition (2026)
- Iron-Deficiency Anemia (Archived), StatPearls (2026)
- Critical Values: critical (panic) limits with adult and pediatric reference intervals, Labcorp (2024)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.