Peripheral Blood Smear Interpretation

A written description of how your blood cells look under a microscope. Labs add it when a CBC result needs a closer look.

AI-verified against cited sources. Awaiting expert reviewAlso called PBS, PBFLOINC 18314-5

What it is

A blood smear is a thin layer of your blood spread on a glass slide and stained so the cells are easy to see. A lab scientist looks at it under a microscope. Sometimes a pathologist, a doctor who studies blood and tissue, looks too. They check the size, shape and color of your red cells, the types of white cells, and the number and look of your platelets. The result is usually a short written comment, not a number. Labs usually make a smear only when the CBC machine flags something to check. At Labcorp, a CBC without a smear comment means a review was not needed.

Why it matters

A smear can confirm an unusual CBC result. It can also show things a machine cannot, such as odd cell shapes, young cells or clumped platelets. It cannot diagnose a condition on its own, so the comment is read with your CBC, your symptoms and other tests. Labs differ on when they review a smear. In one hospital's audit, most pathologist reviews added no new information.

What can affect your result

The lab's review rules: analyzer flags and CBC values decide when a smear is made
Who reads the slide: a laboratory scientist or a pathologist
Sample handling: a difficult draw or a refrigerated tube can cause clumping or clots
Wording and grading of findings can differ between labs
Which results trigger a smear review depends on each lab's own rules, so the same CBC can lead to a smear at one lab and not at another.
The comment reflects what the person reading the slide saw and how they chose to word it, so wording differs between labs.
A difficult blood draw or a refrigerated sample can trap cells and platelets in small clots, which can affect what is seen on the slide.

Describes the blood on the day it was drawn. How quickly it changes depends on the cause.

When to talk to a clinician

If your lab or a pathologist calls you about a result, take the call seriously and follow their instructions. Labcorp says it phones the ordering clinician when a smear suggests a life-threatening blood disorder. Otherwise, bring any smear comment to your clinician at your next visit. Contact your clinician promptly if you also have unusual bleeding or bruising, a fever, or you feel very unwell. Call 911 or your local emergency number for chest pain, fainting or confusion.

Questions you could ask

What did the smear show, and does it change how you read my CBC?
Do I need a repeat CBC or other tests because of it?

Questions people ask

Does a smear comment mean something is wrong?

Not necessarily. Some comments describe small differences in cell size or shape, and some simply say the slide was reviewed. What matters is what the comment says and how it fits with your CBC and symptoms. Your clinician can explain it.

Why do some labs review a smear when others do not?

Each lab sets its own rules for when to look at a slide, based on analyzer flags and CBC values. A 2026 review of 68 studies found that these rules, and how well they work, vary widely.

What do 'Verified by microscopic examination' or 'Slide checked by Pathologist' mean?

Someone looked at your blood slide to confirm what the machine reported. These lines show that a review took place. They do not say whether anything was found, so read the rest of the comment.

Show the science and sources

For clinicians and researchers

The smear is prepared from EDTA blood and read by light microscopy. Review is triggered by analyzer flags, CBC thresholds or a clinician request. The International Consensus Group for Hematology Review proposed a consensus rule set in 2005, and a 2026 systematic review of 68 studies found marked variability in review criteria and in how well flags performed (blast-flag sensitivity 18% to 100%). ICSH recommendations (2015) standardize the nomenclature and grading of red cell, white cell and platelet abnormalities seen by manual microscopy. Labcorp's cascade is: run the CBC, apply flags and rules to decide on a smear, review by a medical laboratory scientist, and pathologist review if criteria are met; the pathologist's comments appear as a footnote or a separate report. One health-system lab (MHCC) sends a slide to a pathologist for triggers that include a WBC above 30,000 per µL, platelets below 35 or above 1000 (x10³/µL), hemoglobin below 6.0 or above 18.0 g/dL, nucleated red cells above 3 per 100 WBC, or any unclassifiable blasts, and it limits repeat reviews for chronic problems to once per 180 days. In an audit of 1,782 laboratory-initiated pathologist reviews at one institution, 89% yielded no additional information and 6% yielded clinically impactful new information. LOINC 18314-5 is the narrative morphology interpretation that Labcorp uses for its 'Hematology Comments' line.

LOINC 18314-5 (primary), 14869-2, 34994-4. Specimen: whole-blood.

Sources

Complete Blood Count (CBC) With Differential, Reflex to Peripheral Smear Review (test 005009), Labcorp
CBC With Differential/Platelet, sample patient report (test 005009), Labcorp
Blood Smear, MedlinePlus, National Library of Medicine
ICSH recommendations for the standardization of nomenclature and grading of peripheral blood cell morphological features, International Journal of Laboratory Hematology (2015)
Performance of Automated Hematology Analyzer Criteria in Detecting Peripheral Blood Smear Abnormalities: A Systematic Literature Review, International Journal of Laboratory Hematology (2026)
Improving laboratory criteria to reduce unnecessary peripheral blood smear reviews by pathologists, Laboratory Medicine (2026)
Complete Blood Count (CBC) with Automated Differential (test code CBCD), MHCC Laboratory (test catalog powered by Mayo Clinic Laboratories)
CBC (includes Differential and Platelets) (test code 6399), Quest Diagnostics

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.