My Alertys Milk Pregnancy Test Results Explained

What is S-N?

S-N is the criteria used to determine the cows pregnancy status as open (not detectably pregnant) S-N less than 0.1 is not pregnant S-N more than 0.25 is pregnant if S-N is between 0.1 and 0.25 the cow should be re-checked.

The S-N value is the comparison in absorbance between each sample and the negative control. This value gives an indication of Pregnancy Associated Glycoproteins (PAGs) present in each sample.

PAG levels in pregnancy.png

If the test is accurate why do I have these results?

It’s possible that you may get unexpected results from your milk pregnancy testing. It’s important to understand the factors at play in understanding your results.
Given the extensive laboratory and trial work completed and the accuracy shown, the likelihood is your test is accurate – but there are other factors that need consideration.
Once received in the laboratory, protocols exist to ensure the test is performing accurately. However, this is reliant on correct sample identification from the cow to the lab. Errors in sample identification may generate unexpected results.


Individual difference, carryover, and early embryonic loss can all result in re-checks. You can expect 3-4% of your results will appear as re-checks in an average herd. An increased number of re-check results (>6%) may indicate an issue requiring investigation.

Individual difference

Cows are like us, we’re all a bit different, and we all react to things in different ways. Expression of PAGs can vary between cows. The cut-off points for pregnancy status are based on averages taken from the extensive testing IDEXX completed to verify the test. This means that pregnant cows that produce below average numbers of PAGs may appear as a re-check. It’s just the nature of individual difference, whilst not a ‘normal’ level in comparison to the average cow this is the normal level for that cow. It may be worth noting cows you suspect of carrying low levels of PAGs through pregnancy so that the next time they can be easily identified.


If a pregnant cow with high levels of PAGs (the ‘pregnancy’ proteins which the Alertys test identifies) is milked before an empty cow and enough milk is carried over to the next sample, it may register on the test. Given the sample amount carried over is small this will most likely appear as a re-check.

Carryover risk evaluation (based on laboratory studies)

Embryonic death and abortion

Embryonic death is common in the early stages of pregnancy. PAGs can circulate in cows for some time after early embryonic death. A re-check could be the result of declining PAGs levels due to embryonic loss. For early embryonic loss, it is estimated PAGs will disappear within 7 days. In the case of abortion, PAGs may be present for a longer period of time. It can take up to 60 days for PAGs levels to drop below the test threshold after foetal loss in late gestation.
Why are my results not what I expected?
The IDEXX milk pregnancy test allows cows to be pregnancy checked as early as 28 days post breeding. Embryonic loss in
the early stages of pregnancy is common and may impact your expected results.

The table below shows the rate of pregnancy loss during gestation from conception through to day 70.
The curve shows that pregnancy loss is more common earlier in gestation.

NZ pregnancy loss in lactating cows.png

Studies that track pregnancy loss through to calving have shown that from day 16 of gestation through
to calving it can be expected that 9 out of 100 cows to lose their pregnancy2. Another study, conducted
in the Waikato, showed that from day 42 of gestation through to calving 6 out of 100 cows lost their

1 Berg D, Meier S Burke C Early embryonic losses in New Zealand seasonal pasture-grazed dairy cattle, Dairy NZ/Ag research.
2 Berg D, 2016, Research finds most pregnancy losses occur in first week, Dairy NZ/Ag research, Dairy NZ Tech Series in brief, Sept; 31: p20-21.
3 McDougall S, Rhodes FM, Verkerk G, 2005, Pregnancy loss in dairy cattle in the Waikato region of New Zealand, NZ Vet Journal Oct;53(5):p279-87.