How to make a peripheral blood smear: the wedge technique, and how to fix a bad smear

Published 13 November 2026 · 15 min read

How to make a blood smear, and how to fix a bad one

The tool for this guide: Open the differential counter

Short answer

Put a 2-3 mm drop of well-mixed EDTA blood about 1 cm from the end of a clean slide. Hold a second slide in front of it at 30-45 degrees, draw it back until the blood runs along its edge, then push it forward in one smooth, steady movement. A good smear covers about two-thirds of the slide and ends in a feathered edge.

The wedge technique, step by step

A wedge smear, also called a push smear, is made by spreading one small drop of blood along a slide with the edge of a second slide held at an angle. The steps below follow the published methods, and each names its source, because the details differ between them in ways that matter.

Mix the sample. Blood collected into EDTA is the usual sample. Adewoyin and Nwogoh's 2014 review asks for a minimum of 10 gentle inversions before the drop is taken, and says samples are best analysed within 2 hours of collection (Ann Ib Postgrad Med 12(2):71-79).
Use clean, unscratched slides, held by the edges. WHO's Basic Malaria Microscopy learner's guide (2nd edition, 2010) tells microscopists to handle slides only by the edges and to discard scratched or chipped ones; a greasy slide spreads the blood unevenly.
Place a 2-3 mm drop near one end. Chonat and colleagues' chapter in the Atlas of Pediatric Hematopathology (Cambridge University Press) gives a drop about 2-3 mm across, placed about a quarter of an inch from the end; the 2014 review places it about 1 cm from the end, and Vu and colleagues' 2021 quality-assessment method about 1 cm from the frosted part of the slide, which keeps the label clear.
Set the spreader at 30-45 degrees in front of the drop. That is the review's range; the Cambridge chapter gives approximately 35-45 degrees, and WHO 45.
Draw the spreader back into the drop and let the blood run right along its edge, as WHO describes, before moving forward.
Push forward in one smooth movement, keeping the angle, until the blood runs out. WHO adds that the edge of the spreader must stay in even contact with the slide. Press no harder than that: in a 2023 study of a mechanical spreader, Nowak and colleagues found the spreader's own weight kept contact, and that even a slight lift made the blood recede and left a band of overlapping cells (arXiv 2304.10200, a preprint).
Air-dry it quickly. The review warns against high humidity, which causes inadequate drying and artefacts around the red cells.
Label the frosted end in pencil, not ballpoint or gel ink, which WHO notes spreads when the film is fixed.
Fix it in methanol, within one hour of making it and no later than four, per the review.
Make two and check them at once. The review recommends two or more slides per sample, assessed immediately, with poor ones discarded and remade.

The spreader is drawn back into the drop and pushed forward at one angle; the finished smear thins from a thick head to a feathered edge, and the counting area, the monolayer, is the narrow band just behind that edge.

What a good smear looks like

A good wedge smear covers about two-thirds of the slide, thins evenly from head to tail and ends in a smooth, rounded feathered edge. The 2014 review asks for two-thirds of the slide length and an oval feathered end; Barbie Papajeski's guide for veterinary teams in Clinician's Brief (updated August 2018) sets the floor at half the slide and describes a rainbow sheen when the smear is held to the light.

Each zone has a job, which is why a smear without one of them is a failed smear rather than a smaller one.

The zones of a wedge smear and what each is used for
Zone	What it looks like	What it is used for
Head (where the drop was)	Thick; cells piled up	Avoided for cells: the review says cell density there is twice that at the tail. Thick areas are scanned at low power for rare cells and parasites
Body	Thinning, cells still overlapping	Low-power scanning
Monolayer	Red cells side by side, touching but not overlapping	Counting and morphology: about one 10× field behind the feathered edge (eClinPath), or 1-2 mm from it (Cambridge atlas)
Feathered edge	The thin, rounded end of the film	Checked at low power for platelet clumps and microfilariae, then avoided: white cells there are over-spread and look larger than they are

The descriptions of the monolayer and feathered edge are from Cornell University's eClinPath, which adds that white cells should be spread evenly, not crowded at the feathered edge. Where to count within the monolayer, and the track to follow, is set out in the guide to the manual differential.

Drop, angle and speed: the three controls

Three things decide how thick and long a wedge smear comes out: the size of the drop, the angle of the spreader and the speed of the push, and the blood itself decides how much of each it needs. The sources agree on the direction of each.

What each control does to a wedge smear
Make this larger	Thickness	Length	Sources
Drop size	Thicker	Can run off the end or sides	WHO 2010; Clinician's Brief 2018; Nowak et al. 2023
Spreader angle	Thicker	Shorter	Adewoyin and Nwogoh 2014; Clinician's Brief 2018; Nowak et al. 2023
Speed of the push	Thicker	Shorter	Adewoyin and Nwogoh 2014; Nowak et al. 2023
Haematocrit of the sample	Thicker	Shorter	Clinician's Brief 2018 (angle advice); eClinPath (polycythaemic samples)

The review states it as a rule: "the faster and steeper the smear, the thicker it is." Nowak and colleagues measured it with a motorised spreader, sweeping angles from 20 to 60 degrees and speeds from 5 to 50 mm per second, and found that increasing the blood volume, the angle or the speed gave thicker, and therefore shorter, smears. In their device, speeds below 15 mm per second spread the cells sparsely and speeds above 20 mm per second left a large zone of overlapping cells.

The speed mistake in the guides

Speed is where published advice disagrees. Clinician's Brief says that moving the spreader too fast "can create a shorter or thinner smear", and too slowly a thicker or longer one. The measured result runs the other way for thickness: a faster push makes the smear thicker and shorter, as both the 2014 review and the 2023 measurements say. A slow push makes a long, thin smear, and eClinPath adds its worst side effect: the white cells are dragged to the end. If a smear is too thick, slow down; if it is too long and thin, speed up.

Why the angle is given as 30, 35-45 or 45 degrees

The sources give different angles because the angle is the setting you change for each sample, not a fixed value.

Spreader angles given by published sources
Source	Angle	Context
Adewoyin and Nwogoh, 2014 review	About 30-45°	Clinical haematology
Vu et al., 2021	30-45°	Smears for external quality assessment
Chonat et al., Cambridge atlas	Approximately 35-45°	Paediatric haematopathology
WHO Basic Malaria Microscopy, 2010	45°	Thin films for malaria, from a finger-prick drop
Clinician's Brief, 2018	30° to start; 30-45° for typical samples; below 30° for high PCV	Veterinary practice

Clinician's Brief makes the adjustment explicit: the angle "may need to be increased" for samples with a low packed cell volume, and decreased below 30 degrees for a high one, as in dehydration or polycythaemia. The review's version is that a steeper, faster smear suits anaemic samples. Start at 30 to 35 degrees for normal blood and move from there.

Faults, their causes and the fix

Almost every bad smear traces to one of four things: the amount of blood, the angle, the push or the glass. The table maps each fault to its cause, from the source that names it, and to the change that fixes the next one. A bad smear is not rescued; the review's advice is to discard it and make another.

Common faults in wedge smears, their causes and fixes
What you see	Likely cause	Fix for the next smear	Source
Thick, short smear; cells overlap everywhere, no monolayer	Drop too big; angle too steep; push too fast; high haematocrit	Smaller drop, lower angle (below 30° for high haematocrit), slower push	WHO; Clinician's Brief; Adewoyin and Nwogoh
Too long; runs to the end of the slide with no feathered edge	Too much blood; push too slow; angle too shallow	2-3 mm drop, even and quicker push, steeper angle	Clinician's Brief; eClinPath
Thin, short film with cells far apart	Drop too small; very anaemic sample	At least a 2 mm drop; steeper, faster smear for anaemic blood	Clinician's Brief; Adewoyin and Nwogoh; eClinPath
White cells crowded at the feathered edge	Push too slow	Remake with a quicker push	eClinPath
Streaks along the smear and many ragged "tails"	Chipped or dirty spreader edge; uneven pressure	New spreader with a clean, straight edge; light, even pressure	WHO; Clinician's Brief
A band of thick, overlapping cells across the smear	Spreader lifted, even slightly, during the push	Keep the edge in even contact and finish in one movement	WHO; Nowak et al.
Holes or patchy, uneven spread	Grease, oil or fingerprints on the slide; moisture; lipaemia	Clean, dry slides held by the edges; lipaemia cannot be fixed by technique	WHO; Clinician's Brief
Drying artefacts around the red cells	Slow drying in high humidity, or water in the fixative	Dry quickly in still, dry air; use absolute methanol	Adewoyin and Nwogoh; Vu et al.
Blue-grey streaks on the stained smear	Smear left unfixed too long; plasma sticks to the glass	Fix within an hour	Vu et al.
Many smudge cells	Fragile cells broken by the shearing of spreading; an old sample	Remake from a fresh sample; for fragile cells, a film with one part albumin to four parts blood	ICSH 2015; eClinPath
Vacuolated neutrophils	Prolonged storage in EDTA	Make smears within 2 hours of collection	ICSH 2015

Two causes in the table are not technique at all. Lipaemia leaves holes however clean the slide, and the fragile cells of chronic lymphocytic leukaemia smudge however gently the smear is pushed, which is why ICSH, in its 2015 recommendations, suggests the albumin film (Palmer et al., Int J Lab Hematol 37:287-303). Knowing those two saves remaking smears that no change of technique can fix.

Worked example: correcting two smears

Smear 1, from a patient with a low haemoglobin. Made at 30 degrees with a 3 mm drop, it runs the full length of the slide and has no feathered edge; the cells in the last third are spread far apart. Thin blood has spread too far. Following the review and Clinician's Brief, raise the angle to about 40 degrees and push a little faster. The next smear stops at about two-thirds of the slide with a rounded edge.

Smear 2, from a dehydrated patient with a high haematocrit. Made at 40 degrees, it is short and dark, with red cells overlapping right to the end. Thick blood has piled up. Lower the angle below 30 degrees, as Clinician's Brief advises for a high packed cell volume, use a 2 mm drop and push more slowly. The next smear thins into a monolayer before the edge.

The check before you stain

Check every smear in the first minute, while remaking it is still cheap: by eye, then at low power. The review asks for the quality of each slide to be assessed immediately. These are the checks, drawn from the sources above.

A five-point check of a wedge smear before staining
Check	Pass	If it fails
Length	Covers about half to two-thirds of the slide	Too long: speed up or steepen. Too short: slow down or flatten
Feathered edge	Present, smooth and rounded, inside the slide	Absent: smear too long. Ragged: chipped spreader
Surface	Even, with no holes, streaks or bands; a sheen in the light	Clean slides, a new spreader, one movement
Monolayer (10× objective)	A band where red cells touch but do not overlap, just behind the edge	Adjust drop and angle for the sample's haematocrit
White cells (10× objective)	Spread evenly, not piled at the edge	Push faster

A smear that passes is ready to fix, stain and count, with each white cell identified by its size, nucleus and cytoplasm. The differential counter then takes one key per cell in the monolayer, along the battlement track, and gives the percentages at 100 or 200 cells.

Time limits from collection to fixing

Make the smear within about two hours of collection and fix it within an hour of making it. The sources set the limits differently, and the strictest is the safe one to follow.

Time limits for blood smears in the sources
Step	Limit	Source
Collection to smear	Best within 2 hours	Adewoyin and Nwogoh 2014; Chonat et al.
Collection to smear	Within 4 hours (quality assessment slides)	Vu et al. 2021
Smear to fixation	Preferably within 1 hour, no later than 4	Adewoyin and Nwogoh 2014
Smear to fixation	Under 6 hours; at 6 hours streaks appeared	Vu et al. 2021

The reasons are cell changes in the tube. The Cambridge chapter warns that delay in preparation can cause cell degeneration, ICSH lists prolonged exposure to EDTA among the causes of vacuoles in neutrophils, and eClinPath notes that smudged cells are more frequent in aged or stored samples.

What this guide does not cover

This guide covers the manual wedge smear from venous EDTA blood for a differential count or morphology. It does not cover staining, which needs its own procedure, automated slide makers, coverslip smears, or the thick films used to find malaria parasites. Two of the sources, Clinician's Brief and eClinPath, are written for veterinary laboratories; the wedge technique is the same, but animal blood varies in how it spreads. Your laboratory's procedure overrides the general figures here, and what a smear shows is for the laboratory and the clinician to interpret.

Simple Online Counter is a free, browser-based set of counters; its differential counter keeps the tally once the smear is made. It does not assess smears or identify cells.

Frequently asked questions

What angle should the spreader slide be for a blood smear?

Between 30 and 45 degrees for most samples. A 2014 review in the Annals of Ibadan Postgraduate Medicine gives about 30-45 degrees, a Cambridge haematopathology atlas 35-45, and WHO's malaria microscopy guide 45. The angle is the adjustment for the sample: a steeper angle makes a thicker, shorter smear, so it is raised for anaemic blood and lowered, below 30 degrees, for blood with a high haematocrit.

How big should the drop of blood be for a smear?

About 2-3 mm across, the figure given in a Cambridge University Press haematopathology atlas; a veterinary technician's guide in Clinician's Brief says no more than 3 mm. A larger drop makes a thick smear that can run off the end or sides of the slide, and a smaller one a thin, short film whose cells are too far apart to be useful.

Why does my blood smear have no feathered edge?

Usually because it is too long: the blood reached the end of the slide before it thinned out. The causes are too much blood, too slow a push or too shallow an angle. Make the next smear with a 2-3 mm drop, push at an even, slightly quicker pace, or raise the angle. Cornell's eClinPath notes that a smear which is too long will lack a feathered edge.

Why are the white cells piled up at the end of my blood smear?

The smear was made too slowly. Cornell University's eClinPath explains that in poorly made smears most of the leukocytes are dragged to the end, usually because the spreader moved too slowly, and the remedy is another smear with a quicker push. Large cells such as monocytes collect at the feathered edge, so a count made on that smear would be biased.

What causes holes in a blood smear?

Grease or dirt on the slide is the usual cause. WHO's malaria microscopy guide says films on a greasy slide spread unevenly, and Clinician's Brief lists dirt, oil, fingerprints, moisture, lipaemia, an aged sample and clumped platelets. Use new clean slides held only by their edges. A lipaemic sample can leave holes however good the technique.

How soon after collection should a blood smear be made?

As soon as possible, and ideally within two hours of collection, the window given by both a 2014 peer-reviewed review and a Cambridge haematopathology atlas. Delay degenerates cells and causes storage artefacts: ICSH lists prolonged EDTA exposure as a cause of neutrophil vacuolation. Once made and air-dried, the smear is best fixed within an hour and no later than four.

What is the feathered edge of a blood smear?

The thin, rounded far end of a wedge smear, where the film runs out. Cells there are spread too thin to judge, but it is scanned at low power for platelet clumps and parasites. The area used for counting and morphology, the monolayer, lies just behind it: about one 10× field back in Cornell's eClinPath, or 1-2 mm from the edge in a Cambridge atlas.

How do you make a blood smear from anaemic blood?

Make it steeper and faster. Thin, anaemic blood spreads into a long, thin film, so a 2014 review advises a steeper, faster smear for anaemic samples, and Clinician's Brief advises raising the angle when the packed cell volume is low. For blood with a high haematocrit, do the opposite: lower the angle below 30 degrees and use a smaller drop.