You read that the fertile window starts around 8 to 10 ng/mL. Your analyzer’s chart says to breed at 15 to 35. Your friend’s vet quotes something else again. Everyone sounds confident, and the numbers do not agree.
They can all be right at the same time, because progesterone thresholds are analyzer-specific. Here is why — and, more usefully, which parts of a progesterone chart you can carry between instruments and which parts you absolutely cannot.
- Different assay technologies give systematically different numbers on the same sample. This is expected, not a defect.
- Absolute values do not transfer between instruments. A threshold from one analyzer’s chart is meaningless on another.
- Day counts do transfer. Breeding on days 4 and 6 after the LH surge is instrument-independent.
- Use the reference range published for your analyzer, and stay on one platform for the whole cycle.
The same hormone, measured four different ways
Progesterone in canine serum is a single molecule with a single true concentration. What differs is how each instrument infers that concentration.
| Detection principle | Typical platforms |
|---|---|
| Radioimmunoassay (RIA) | The historical reference method, now rare |
| Chemiluminescence (CLIA / ECLIA) | Reference-laboratory analyzers adapted from human clinical chemistry |
| Enzyme-linked fluorescence (ELFA) and fluorescence enzyme immunoassay (FEIA) | Benchtop laboratory analyzers |
| Fluorescence immunoassay, lateral flow (FIA) | Point-of-care veterinary analyzers, including the PETlife Pro-DX |
Each of these uses different antibodies, different reagents, different calibration curves, and a different physical signal. The resulting disagreement has been measured directly. Comparing the Siemens Immulite chemiluminescence assay against the Coat-A-Count radioimmunoassay on paired samples from the same bitches, Nöthling and De Cramer found that Immulite values averaged just 85% of the RIA result, with a wide scatter around that mean (95% confidence interval 58% to 112%), and that 88% of Immulite readings were lower than their RIA counterparts.
A 15% average shift is not a rounding error in a range where the whole decision lives between 1 and 10 ng/mL. And the confidence interval is the more important number: on an individual sample the gap could be 42% low or 12% high.
Running the same progesterone sample on different systems produces different numbers. This is expected. The measurement methods and reagents are not the same, so the results are not the same.
This is why a manufacturer publishes a reference range tied to its own assay, and why textbook thresholds derived on a laboratory analyzer are not drop-in replacements for it. Validation studies are run per platform for the same reason: Milani and colleagues, writing in the Journal of Veterinary Diagnostic Investigation in 2022, had to establish the analytical performance of a fluorescence enzyme immunoassay specifically on canine serum before it could be used for ovulation timing, reporting intra-assay coefficients of variation of 2.6% to 5.2%. Good performance on one instrument says nothing about the numbers another instrument will report.
Progesterone thresholds don’t transfer; day counts do
The most useful mental model is to split a progesterone protocol into two layers and treat them completely differently.
The right-hand column is anchored to events, not to numbers. That is what makes it portable. Once your analyzer has told you where Day 0 is on its own scale, the calendar takes over and the instrument stops mattering.
A worked example
Suppose you are running canine progesterone on a PETlife Pro-DX. The published specification for the cProg test gives a linear range of 1 to 50 ng/mL and a reference range of 15 to 35 ng/mL, on 75 µL of sample with results in about 15 minutes.
Meanwhile, the interpretation table in Kurt De Cramer’s widely used reference on simplifying the canine reproductive cycle — built on laboratory assay values — places the fertilizable period at 8 to 10 ng/mL and above. Those two statements look contradictory. They are not, because they are two different rulers describing the same event: the point in the cycle at which the oocytes have matured. Read each against its own scale and both point at the same days.
The error to avoid
Reading 9 ng/mL on a point-of-care analyzer, recalling that “8 to 10 means fertilizable” from an article written around a different platform, and breeding that afternoon. The number came from one ruler and the rule came from another. Check where you are on your analyzer’s chart, and cross-check it against the day count from Day 0.
What to do about it, practically
- Find the reference range for your exact test and instrument. Not the analyzer brand in general, and not a chart someone posted for a different device. If it is not in the kit insert, ask the manufacturer.
- Stay on one platform for the entire cycle. Since timing depends on detecting a rise, an assay that reads consistently is usable even if its absolute values differ from a reference laboratory. Mixing platforms mid-cycle destroys the trend.
- Record the analyzer alongside every result. A value from two seasons ago is only comparable to today’s if you know it came from the same instrument.
- Anchor on Day 0 and count days. This is the layer that survives any platform change.
- Check the unit. ng/mL and nmol/L differ by a factor of about 3.18, and a number that looks post-ovulatory in one unit can be a just-surged value in the other.
For the Wondfo cProg test specifically, the published specifications and reference range are listed on the PETlife Pro-DX product page, and technical questions can go to the PETlife support team at vethelp@wondfousa.com or 1-866-375-HELP (4357). Asking the manufacturer for the range that matches your kit is a reasonable request, and a better source than a third-party chart.
The same four events on a miniVIDAS, an IDEXX chart, and the Pro-DX
Many reproduction clinics hand out a two-column card that lists miniVIDAS values next to IDEXX values, row by row, with the LH surge, ovulation, first breeding and the frozen-semen implant marked down the side. The best-known version comes from the International Canine Semen Bank in Colorado (ICSB-CO) and dates from 2004. Breeders who move to a PETlife Pro-DX often ask where its numbers would sit on that card. The table below answers that by anchoring all three columns to the same events rather than by converting any number.
| Event on the cycle | miniVIDAS ICSB-CO chart, 2004 | IDEXX ICSB-CO chart, 2004 | PETlife Pro-DX ★ Finecare Vet / Pro-DX timing chart, 2025 |
|---|---|---|---|
| LH surge, Day 0 “Looking for a significant rise” | ≈ 2.0 band 1.5–3.5 | ≈ 1.5 band 1.5–2.5 | ≈ 2–4 distributor guidance; the timing chart itself starts at 8 and says “below 8, test again in 3 days” |
| Ovulation, Day 2 “Breed in 48 hours” | ≈ 8.5 band 8.0–10.5 | ≈ 4.5 band 4.5–5.5 | 8–15 · “Estimated ovulation” test again the next day to confirm the jump; if still 8–15, she has not ovulated yet |
| First breeding, Days 4–6 “First breeding ideal” | ≈ 17 band 15–20.5 | ≈ 9 band 8–11 | 15–40 · “Fertile range” AI or natural tie 1–3 days after ovulation; inseminate twice within the range |
| Frozen-semen implant, Days 6–7 “Surgical implant of frozen semen ideal, or last time to breed” | ≈ 36.5 band 33–40 | ≈ 19.5 band 18–21.5 | Still within 15–40 TCI or surgical 3–5 days after ovulation; the chart gives no separate value and warns that the chance of pregnancy falls after 4 days post-ovulation |
| Retest cadence | — | — | Below 8: retest in 3 days · 8–15: every 2 days · 15 and above: every day |
| What the chart is anchored to | The LH surge. Days are counted from the rise. | The ovulation date. The chart asks you to pin down ovulation with a next-day test, then count breeding days from there. | |
How to read this table
Read across a row, never down a column into another analyzer’s chart. The miniVIDAS and IDEXX columns are one clinic’s working card, compiled from its own paired results in 2004, and the “IDEXX” it refers to is the IDEXX reference-laboratory assay of that time, not the Catalyst One bench analyzer sold today. The spread between those two columns is wider than the published validation work would predict, which is a reminder that a clinic card is a practical tool, not a validation study.
The Pro-DX column is not derived from the other two. It is the manufacturer’s own timing chart, which is written around the ovulation date rather than the LH surge. That is why its “first breeding” and “frozen” rows share one range: on this chart the day count from ovulation does the work, and the number only confirms that she is still inside the fertile range. Follow the chart supplied with your kit lot, since the manufacturer notes the ranges can shift slightly between lots.
The practical translation for someone switching from a miniVIDAS card to a Pro-DX: the moment you would have circled on the card as “ovulation, breed in 48 hours” is the moment the Pro-DX reads 8 to 15 ng/mL and jumps clear of that band the next day. The “first breeding ideal” row on the card corresponds to the Pro-DX result entering the fertile range at 15, one to three days after that jump. And the “frozen implant” row is not a number on the Pro-DX at all. It is three to five days after the confirmed ovulation, with the result still inside 15 to 40.
Why point-of-care testing still wins on timing
A fair question follows from all of this: if reference-laboratory chemiluminescence correlates most closely with mass spectrometry, why not simply send everything out?
Because ovulation timing is not an accuracy problem. It is a sampling frequency problem. As De Cramer puts it, turnaround time impedes the usefulness of commercial laboratory assays precisely in the cases where an immediate clinical decision is needed — which is the reason point-of-care progesterone exists at all. The decisions that matter depend on catching a rise as it happens and confirming it the next day, and a highly accurate result that arrives 36 hours later can be worth less than a slightly different scale you can read this morning and again tomorrow.
What matters is not that two instruments produce identical absolute numbers, but that the one you use generates internally consistent, clinically interpretable results across the concentrations where the decisions live. That is a lower bar than laboratory equivalence, and a more relevant one.
One caution about semi-quantitative kits
Everything above assumes a quantitative result. Kits that report a band or a category rather than a number are a different proposition: De Cramer notes that semi-quantitative progesterone tests proved much less useful and reliable than quantitative assays returning an actual numerical value. They can answer “has anything started yet”, which is genuinely useful early in the cycle, but they cannot distinguish 2.2 from 3.4 ng/mL — and that distinction can move Day 0 by a full day.
Frequently asked questions
Why does my analyzer’s chart show higher numbers than the articles I read?
Because the chart is calibrated to that assay. Different detection principles and reagents produce systematically different values on the same sample — Nöthling and De Cramer measured chemiluminescence readings averaging 85% of radioimmunoassay values on paired canine samples. Both charts can be correct for their own instrument while disagreeing numerically.
Is there a conversion factor between analyzers?
Not one you should rely on. Published cross-platform conversion equations for canine progesterone are not generally available, and the relationship is not a simple constant: the 95% confidence interval in the Immulite-versus-RIA comparison spanned 58% to 112%. Stay on one instrument rather than trying to convert.
Which numbers from a breeding article can I actually use?
The day counts. Ovulation roughly 2 days after the LH surge, eggs fertilizable roughly 2 days after that, breeding on days 4 and 6 after the surge for fresh semen and days 6 and 7 for frozen or chilled, whelping about 65 days after the surge. These are anchored to events, not to a scale.
What is the reference range for the Wondfo cProg test?
The published linear range is 1 to 50 ng/mL and the listed reference range is 15 to 35 ng/mL, on 75 µL of sample with results in about 15 minutes. Confirm against the insert for your specific kit and version, since whole blood and serum or plasma kits are supplied separately.
My clinic gave me a miniVIDAS-versus-IDEXX card. Where does the Pro-DX sit on it?
Nowhere directly, because the Pro-DX chart is anchored to the ovulation date rather than to the LH surge. The event the card marks as “ovulation, breed in 48 hours” (about 8.5 on the miniVIDAS column, about 4.5 on the IDEXX column) is the Pro-DX “estimated ovulation” band of 8 to 15 ng/mL, confirmed by a jump the next day. The card’s “first breeding ideal” row corresponds to the Pro-DX result entering the 15 to 40 fertile range one to three days later, and the card’s frozen-semen row corresponds to three to five days after confirmed ovulation, not to a separate number. See the three-analyzer table above.
Sources
Nöthling JO, De Cramer KGM. Comparison of progesterone assay by chemiluminescence or radioimmunoassay for clinical decision-making in canine reproduction.
Milani C, Boscato EL, Gabai G, et al. Analytical and clinical performance of a fluorescence enzyme immunoassay for progesterone and determination of ovulation day in bitches. J Vet Diagn Invest. 2022.
De Cramer K. Simplifying the Canine Reproductive Cycle Using the Concentration of Progesterone. IDEXX Laboratories, CLD-12211-00.
Wondfo USA. PETlife Pro-DX Immunofluorescence Analyzer, published test menu specifications.
International Canine Semen Bank – Colorado (Brighton Animal Clinic). Progesterone value comparisons for ovulation timing: miniVIDAS and IDEXX. Clinic comparison chart, ICSB-CO 08/04.
MR Diagnostic Services / CanineP4. Finecare Vet & Pro-Dx Progesterone Timing Chart, 2025 update (FM01.30116.03). Estimated ovulation 8–15 ng/mL; fertile range 15–40 ng/mL; AI or natural tie 1–3 days and TCI or surgical 3–5 days after ovulation.
Keep reading
Product specifications cited here are those published by the manufacturer at the time of writing; always confirm against the insert supplied with your kit. This article is educational and is not a substitute for veterinary advice. Breeding decisions and result interpretation should be made in consultation with your veterinarian.