McMaster faecal egg count: how to count the eggs and which multiplication factor to use

Where the multiplication factor comes from

The McMaster factor is one divided by the grams of faeces in the chambers you read, which works out as total volume of suspension ÷ grams of faeces ÷ millilitres counted. Each chamber of a standard McMaster slide holds 0.15 mL under its grid, as the University of Minnesota's Clinical Veterinary Diagnostic Laboratory states, so reading both chambers examines 0.3 mL.

Written out, with the 3 g in 45 mL recipe that gives the familiar 50:

1. Grams per millilitre of suspension. 3 g ÷ 45 mL = 0.0667 g per mL, or 15 mL per gram.
2. Faeces in the chambers. 0.3 mL × 0.0667 g per mL = 0.02 g.
3. The factor. 1 ÷ 0.02 g = 50. Each egg seen stands for 50 eggs per gram.

Martin Nielsen sets out the same sum in Equine Veterinary Education for 4 g in 60 mL: "(60/4)/0.3 = 50 EPG", and adds the caveat that matters most. The factor "is not determined in a study, but is a purely theoretically calculated number": it assumes the eggs are perfectly mixed and that none are lost, which, he writes, is never the case (Equine Vet Educ 34(11):584-591, 2022).

The factor for each published recipe

Different recipes give different factors, 16.66 to 100, and every one of them follows from the same formula. The table shows each published recipe and checks its factor against the formula.

The sources: Moredun's DIY egg counting page; the AAEP Internal Parasite Control Guidelines, Appendix A; the Minnesota text linked above, whose worked example of 5 + 2 strongyle eggs gives (5 + 2) ÷ 0.3 × 60 ÷ 2 = 700 EPG; Vadlejch and colleagues' comparison of McMaster modifications in Parasitology Research (2011); and Camila Queiroz's University of Calgary handout for sheep and goat producers. The three-chamber slide in that handout is sold with the Paracount-EPG kit, whose maker describes it as having three times the sensitivity of the standard two-chamber slide, which is what a factor of 50 ÷ 3 = 16.66 needs.

Concentration methods: the same formula with one more step

Concentration McMaster methods spin the eggs down and resuspend them in less fluid, and the formula still holds. In Roepstorff and Nansen's method as Vadlejch and colleagues describe it, 4 g of faeces go into 56 mL of water (60 mL in all), a 10 mL tube of the suspension is centrifuged, and the sediment is resuspended in 4 mL of flotation fluid. That tube held 10 ÷ 60 × 4 g = 0.667 g of faeces, now in 4 mL, so 0.3 mL holds 0.05 g and the factor is 20, the figure the paper uses. The same check on Zajíček's method, 1 g brought into 10 mL of flotation fluid and two chambers read, gives 10 ÷ 1 ÷ 0.3 = 33, again the paper's figure.

Total volume, not the fluid you added

The formula needs the total volume of the suspension, faeces included, and a common slip is to use the fluid added instead. The AAEP recipe avoids it by filling to a line: add 26 mL, then faeces "to bring the volume up to 30 ml". Moredun's recipe adds 42 mL to 3 g, and its factor of 50 treats the result as 45 mL, counting about a millilitre of volume per gram of faeces. Using 42 mL instead gives 42 ÷ 3 ÷ 0.3 = 46.7, 7% lower; for the AAEP recipe, 26 ÷ 4 ÷ 0.3 = 21.7 against 25, 13% lower. Use the factor your protocol states; if you build your own recipe, measure the total volume after mixing.

How to run a McMaster count

Weigh, suspend, strain, fill both chambers while the suspension is still mixed, wait for the eggs to float, then count every egg inside the grids at 100× magnification. The steps follow the AAEP's 2024 Appendix A, with the Moredun recipe's figures in brackets where they differ.

1. Weigh the faeces. 4 g (Moredun: 3 g). Without a scale, the AAEP's method adds faeces to 26 mL of fluid until it reaches the 30 mL mark, which is about 4 g.
2. Add the flotation fluid and mix well. 26 mL to make 30 mL (Moredun: 42 mL saturated salt).
3. Strain through cheesecloth, a gauze square or a tea strainer, and mix again.
4. Fill the chambers at once. Draw about 1 mL while the suspension is still moving and fill one chamber, then mix and fill the other. The AAEP says to do the straining and filling together, without letting the sample sit, because eggs start to float straight away. Refill a chamber that has air bubbles.
5. Let the slide stand for two to five minutes so the eggs rise to the grid; with sodium nitrate the AAEP allows up to 60 minutes before crystals become a risk.
6. Count both chambers with the 10× objective (100× in total), focused on the top layer, one grid at a time. Record each chamber and each egg type separately.
7. Multiply each egg type's total by the factor.

Eggs on the grid lines

The rule for eggs touching the lines differs between protocols. The AAEP counts only eggs with more than half of their area inside the outer lines of the grid; the Minnesota text excludes eggs that touch the grid lines or lie outside the chamber; the Calgary handout says to count all eggs within the entire grid system. Pick the rule your protocol names and use it on every chamber, so a count before treatment and one after are made the same way.

Worked examples

Three counts, each multiplied by its own recipe's factor.

A cattle sample, factor 50

Moredun recipe, 3 g in 42 mL. Chamber 1 shows 14 strongyle-type eggs and 1 Nematodirus egg; chamber 2 shows 9 strongyle-type eggs and none.

• Strongyle-type: (14 + 9) × 50 = 1,150 EPG.
• Nematodirus: (1 + 0) × 50 = 50 EPG, the smallest result above zero this recipe can give.

A horse sample, factor 25

AAEP recipe, 4 g made up to 30 mL. Chamber 1: 22 strongyle eggs; chamber 2: 18 strongyle eggs and 1 ascarid egg.

• Strongyle: (22 + 18) × 25 = 1,000 EPG.
• Ascarid: 1 × 25 = 25 EPG.

A sheep sample on a three-chamber slide, factor 16.66

Calgary recipe, about 2 g to 30 mL, three chambers read: 7, 5 and 6 eggs. (7 + 5 + 6) × 16.66 = 299.9, reported as 300 EPG. A standard two-chamber slide reads a third of that volume, so the same suspension would show about 6 eggs, and 6 × 50 is also 300 EPG on average. The difference is the chance in it: 18 eggs counted carry a relative error of about 24%, 6 eggs about 41% (the precision section explains the arithmetic).

What a zero count means: the detection limit

A zero means no egg was in the fraction of a gram you looked at, so it is reported as below the factor, such as under 50 EPG, never as zero. With the factor of 50, the chambers held 0.02 g; an animal can shed eggs at a real rate and still leave none in that sliver.

How often that happens can be worked out if the eggs are assumed to be evenly mixed, so that the number in the chambers follows a Poisson distribution. The expected number of eggs in the chambers is the true EPG divided by the factor, and the chance of seeing none is e raised to minus that number.

These are the best case. Vadlejch and colleagues point out that parasite eggs in naturally infected faeces follow a negative binomial distribution, clumped rather than evenly spread, and Nielsen notes that eggs are always lost in processing; both make a false zero more likely than the table shows. In their spiked-sample test, the concentration method with a factor of 20 detected eggs at its assumed 20-egg limit in only 70% of samples. Nielsen's practical advice is that if reliably finding low counts matters, a more sensitive technique should be used.

How precise a McMaster count is

A McMaster result is only as precise as the number of eggs actually counted, not the EPG it reports. If the counted eggs follow a Poisson distribution, their standard deviation is the square root of the count, so the relative error is 1 ÷ √n. Twenty-three eggs counted, the cattle example above, carries a relative error of about 21%; 40 eggs, about 16%; 4 eggs, 50%. The same arithmetic governs plate counts and the 100-cell blood count.

The World Association for the Advancement of Veterinary Parasitology built its 2023 guideline for the faecal egg count reduction test (FECRT) on this point. Kaplan and colleagues write that precision improves as more animals are tested and as the multiplication factor falls, "i.e. the number of eggs counted under the microscope increases", and they replaced the old minimum mean EPG with a minimum total of eggs counted before any factor is applied (Vet Parasitol 318:109936). If too few eggs are seen, the guideline's remedy is to read a second count, or an additional chamber, from every animal, and to read the same number after treatment.

The table shows what a factor of 50 costs. A flock averaging 400 EPG gives only 8 eggs per animal on a factor-50 slide, so 14 animals must be sampled for the clinical protocol; on a method with a factor of 25 the same flock gives 16 eggs per animal, enough for 11.

Counting by egg type without losing the tally

Each egg type is counted and multiplied separately, so the tally needs one column per type, kept per chamber. Moredun's page counts strongyle and Nematodirus eggs; the AAEP counts strongyle, ascarid and, if present, tapeworm eggs, and only notes whether Strongyloides and Eimeria leuckarti are present; the Minnesota text records strongyle-type eggs and coccidia, the two its test is validated for.

Simple Online Counter is a free, browser-based set of counters that keeps its counts in your browser's own storage. Its multi counter keeps one named counter per egg type, "Strongyle ch1", "Strongyle ch2", "Nematodirus", with undo for a double tap, and exports them as a CSV; the tally counter does a single type with one tap per egg. Neither identifies eggs: telling a strongyle-type egg from a Nematodirus egg is your judgement, and the factor is applied afterwards.

McMaster against other egg counting methods

McMaster is a dilution method, fast and cheap, but not the most sensitive, and its results are not interchangeable with other methods'. The method descends from Gordon and Whitlock's 1939 paper, "A new technique for counting nematode eggs in sheep faeces", in the Journal of the Council for Scientific and Industrial Research (Australia); the WAAVP guideline cites the UK Ministry of Agriculture's 1986 manual for the McMaster method in use today.

The Wisconsin row is the lesson in the table. A factor of 1 sounds ideal, but the WAAVP guideline explains that centrifuge-and-coverslip methods perform with substantially lower accuracy and precision than counting-chamber methods such as McMaster and Mini-FLOTAC. A low factor helps only when the rest of the method is sound, which is also Vadlejch and colleagues' conclusion. And the Minnesota text adds the rule for records: results from two techniques "are not interchangeable", so a before-and-after comparison must use one method throughout.

What this guide does not cover

This guide covers the counting and the arithmetic of the McMaster technique. It does not cover identifying eggs, which species of parasite matter, the EPG at which treatment is worthwhile, or how to design and interpret a faecal egg count reduction test beyond the egg totals above. Those depend on the host species, its age, the farm and the drug, and belong to your veterinarian and the guidelines cited here. Use the factor your laboratory's or kit's protocol states, and keep the method the same from one count to the next.