Safety first: only ever use your own blood, collected with a new, sterile, single-use lancet. Never share lancets or reuse them. If you have a bleeding disorder, take blood-thinning medicines, or have any health concerns, skip the finger prick and use a prepared slide instead. A home blood smear is for learning, not for diagnosing anything. If you notice something that worries you, talk to a doctor rather than relying on your own slide.
A blood smear is a thin film of blood spread across a microscope slide, dried, and usually stained. Done well, it shows a single layer of red blood cells with white blood cells and platelets scattered among them. It’s the same basic method labs use for manual blood counts, and it’s very achievable at home with two slides and a little practice.
What you need
- Clean glass microscope slides, ideally ones with a frosted end for labelling
- A second slide with a smooth, clean edge to use as the spreader
- A sterile, single-use lancet (sold at pharmacies for blood glucose testing)
- Alcohol wipes, cotton wool and a plaster
- A pencil for labelling
- For fixing and staining (optional but recommended): methanol and a blood stain (see below)
- A compound microscope. 400x shows the cells; a 100x oil-immersion objective (1000x total) shows much more detail
- A sharps container, or a rigid lidded container for used lancets
Getting a drop of blood safely
- Wash your hands with warm water and soap and dry them. Warm hands bleed more easily, so run them under warm water first if they’re cold.
- Clean the fingertip with an alcohol wipe and let it dry completely. Wet alcohol can affect the blood cells and makes the drop spread.
- Prick the side of the fingertip, not the pad, with the lancet. The side is usually less painful. Follow the instructions that came with the lancet.
- Wipe away the first drop with clean cotton wool if you like. Many lab methods do this, because the first drop can be mixed with tissue fluid.
- Let a fresh drop form. Gently massage from the base of the finger towards the tip. Avoid hard squeezing, which can dilute the blood with tissue fluid.
- Touch the slide to the drop to collect it (see below), then press cotton wool on the finger and apply a plaster.
- Dispose of the lancet immediately in a sharps container. Many pharmacies can advise on safe disposal.
Alternatives to a finger prick:
- Commercially prepared blood slides of human, frog, bird or fish blood are inexpensive and already stained. They’re the best option for children and classrooms.
- Animal blood from a fresh fish from a fishmonger can work, but it clots quickly and is often of variable quality. Interestingly, fish, frog and bird red blood cells have a nucleus, unlike mammal red cells.
Making the smear: the wedge technique
Work quickly once the blood is on the slide. It starts to clot within a minute or two.
- Place a small drop of blood near one end of the slide, about 1 cm from the frosted end. It should be small: roughly 2–3 mm across. A drop that’s too big is the most common reason for a thick smear.
- Hold the spreader slide at an angle of about 30–45 degrees to the first slide, with its edge resting on the slide just in front of the drop (on the side away from the frosted end).
- Pull the spreader back into the drop. The blood will spread along the edge of the spreader by capillary action. Wait a second until it’s spread almost all the way across.
- Push the spreader forward in one smooth, steady movement, at a moderate speed, along the length of the slide. Keep the angle constant and the pressure light. Let the blood follow the spreader; don’t push the blood ahead of it.
- Don’t stop or lift until the blood has run out. You should be left with a smear that covers roughly half to three-quarters of the slide and ends in a thin, curved feathered edge.
- Air-dry immediately. Wave the slide gently in the air, or hold it in front of a cool fan, so it dries within seconds to a minute. Quick drying keeps the cells’ shapes. Don’t use heat.
Make several smears. Even experienced lab staff often make two or three to pick the best.
What a good smear looks like
Hold the dry slide up to the light. A good smear:
- Has a gradual change from thicker (near the drop) to thin (near the feathered edge).
- Has smooth sides that don’t touch the edges of the slide.
- Ends in a smooth, slightly rounded feathered edge, without streaks or tails.
- Has no holes, ridges or lines across it.
Under the microscope, the best area to examine is the “monolayer” just behind the feathered edge. There the red cells sit side by side, just touching, but not piled up. At the very thick end they overlap in stacks. Right at the feathered edge they’re flattened and distorted, and white cells tend to pile up there.
Troubleshooting the smear
| Problem | Likely cause | Try |
|---|---|---|
| Smear too thick and short | Drop too big, angle too steep, or pushed too fast | Smaller drop; lower the angle (nearer 30°) |
| Smear too thin and long, no feathered edge | Drop too small, angle too shallow, or pushed too slowly | Raise the angle (nearer 45°); push a little faster |
| Lines or streaks along the smear | Dirty or chipped spreader edge; blood starting to clot | Clean or replace the spreader; work faster |
| Ridges or bands across the smear | Hesitating or jerky movement | One smooth, continuous push |
| Holes or bare patches | Greasy or dirty slide | Clean slides with alcohol and handle them by the edges |
| Smear runs to the slide edges | Blood spread too far along the spreader | Push sooner, before the blood reaches the spreader’s corners |
| Cells look shrunken and spiky | Slow drying | Dry faster by waving or using a cool fan |
Drying, labelling and fixing
- Label the frosted end in pencil with the date and what’s on it. Pencil survives methanol and stains; many inks don’t.
- Fix the smear if you’re going to stain it. Fixing preserves the cells and stops them washing off. The standard fixative is methanol: dip the dry slide in methanol, or flood it with a few drops, for a minute or two, then let it air-dry. Many stain kits include a fixative as their first step.
- Methanol is toxic and flammable. Use it in a well-ventilated room away from flames, wear gloves, and keep it away from children. Store it in its original labelled container.
An unfixed, unstained smear is still interesting to look at, but the cells are pale and hard to tell apart.
Staining options
| Stain | What it does | Notes for amateurs |
|---|---|---|
| Three-step “quick” stain kits (Romanowsky-type, sold under various brand names) | Fixative, then a red stain, then a blue stain; colours red cells pink and white cell nuclei purple | The easiest option; dip the slide in each solution for a few seconds each, following the kit instructions |
| Giemsa | Classic blood and parasite stain; nuclei purple, red cells pinkish | Usually diluted in buffered water and left on for a longer time; follow the supplier’s instructions |
| Wright’s or Wright-Giemsa | Standard clinical blood stain | Applied to the slide, then diluted with buffer on the slide; timing takes practice |
| Field’s stain | Quick stain, often used for thick and thin films | Two solutions, short dips |
| Methylene blue | Stains nuclei of white cells blue; red cells stay pale | Simple and easy to get, but it won’t show granules or cytoplasm colours well; see our methylene blue staining guide |
A general method for most stains:
- Fix the smear (if your stain doesn’t include a fixing step).
- Apply the stain, or stains, for the times in the instructions.
- Rinse gently with water, or with the buffer the kit specifies. Tap water can change colours with some stains.
- Stand the slide upright to air-dry. Don’t blot the smear side.
A stained, dry smear can be viewed as it is. For oil immersion, put a drop of oil directly on the dry smear. For a slide you want to keep, add a coverslip with a resin mounting medium; see how to make permanent microscope slides.
Viewing: where to look and what magnification
- Start at low power (100x) to check the smear and find the monolayer area just behind the feathered edge.
- Move to 400x to see red cells and recognise white cells by their stained nuclei.
- Use 1000x with oil immersion for detail: nuclear shapes, granules and platelets. Put one drop of immersion oil on the smear, swing the oil objective into place, and focus with the fine focus only.
What you’ll see
Sizes are approximate.
| Cell | Approximate size | What it looks like in a typical stained smear |
|---|---|---|
| Red blood cell (erythrocyte) | About 7–8 µm across | Pink, round disc with a paler centre (the dip in the middle of the biconcave shape); no nucleus in mammals |
| Neutrophil | Larger than a red cell | Nucleus with several lobes (often 3–5) joined by thin strands; pale cytoplasm with fine granules; usually the most common white cell |
| Lymphocyte | Often similar in size to a red cell or a bit larger | Round, dark nucleus filling most of the cell, with a thin rim of pale blue cytoplasm |
| Monocyte | The largest white cell | Large, often kidney- or horseshoe-shaped nucleus; grey-blue cytoplasm |
| Eosinophil | Larger than a red cell | Usually two-lobed nucleus; cytoplasm packed with orange-red granules |
| Basophil | Larger than a red cell | Dark purple granules that often hide the nucleus; rare |
| Platelets | Much smaller, about 2–3 µm | Small purple fragments, often in little clumps, between the red cells |
White blood cells are far less common than red cells. You may scan many fields to find each type, and basophils are rare enough that you may not find one at all.
Remember that cell counts and appearances vary between people and with how the smear was made and stained. Don’t try to read health information from a home slide.
Artefacts to watch for
Many odd things on a home smear are caused by preparation, not by the blood:
- Refractile spots or “holes” in red cells: often caused by moisture, for example from water in the methanol or slow drying.
- Spiky (crenated) red cells: often from slow drying or a smear that’s too thick.
- Stain precipitate: dark granules or crystals scattered over the slide, which can look like bacteria or parasites. Filtering stain and rinsing well helps.
- Smudged or burst cells: fragile cells broken during spreading.
- Fibres, dust and scratches on the slide.
Our guide to microscopy artifacts covers these and more, and explains how to tell them from real features.
Quick alternative: an unstained wet mount
If you just want to see living red cells, put a tiny drop of blood on a slide with a drop of saline (0.9% salt solution, such as sterile contact-lens saline) and add a coverslip. At 400x you’ll see red cells floating and tumbling, sometimes stacking like coins. Try plain water instead of saline and watch them swell and burst; try strong salt water and they shrivel and become spiky. It’s a good demonstration of osmosis. Use the bubble-free wet mount technique for a clear view.
Clean-up and disposal
- Lancets go straight into a sharps container. Never put loose lancets in household rubbish.
- Slides with blood should be soaked in a disinfectant, such as diluted household bleach made up according to the label, before washing or disposal. Wear gloves.
- Stains and methanol: follow the disposal instructions on the product or its safety data sheet.
- Wipe down your work surface with disinfectant when you’ve finished, and wash your hands.
If you enjoyed this, a good next step is learning to see cells in solid tissue. Our beginner’s guide to histology explains how tissue slides are made and what to look for.