A breeder buys test kits from a marketplace listing because they are cheaper. The analyzer accepts them. It returns a number, in the right units, inside the range you would expect. Nothing on screen suggests a problem. The breeding is timed from that number, and it is wrong.
Test kits are not a generic consumable. They carry the calibration, and a kit built for a different market or a different sample type reads on a different scale. The failure is silent, which is what makes it dangerous.
- The calibration lives in the kit, not in the analyzer. Changing kit changes the scale you are reading.
- Kits are released per market, and the packaging differs. Use the version authorised for your region.
- Whole-blood and serum kits are separate products. A sample prepared for one is not valid on the other.
- A mismatched kit does not raise an error. It returns a plausible number, which is why this is worth checking before every new box — not after a result looks strange.
Why the kit carries the calibration
A fluorescence immunoassay does not measure hormone directly. It measures a signal, and then converts that signal to a concentration using a curve established for that reagent formulation and that manufacturing lot. The analyzer reads the signal. The kit supplies the curve.
This is why the instrument accepts a kit it was never validated against without complaint: it is doing exactly what it was built to do, applying the conversion it was handed. If that conversion belongs to a different product, every number downstream is shifted.
The same argument applies between instruments, and we make it at length in why your analyzer’s chart doesn’t match the textbook. A threshold belongs to the assay that produced it. Swapping the reagent is swapping the assay.
Three ways kits get mixed up
| Situation | What goes wrong | What to do |
|---|---|---|
| Kit sourced from a marketplace or grey import | Released for another market, calibrated on another scale. Runs without error. | Buy only kits authorised for your region, from a channel that can tell you the lot history. |
| Whole-blood kit run with serum, or the reverse | The preparation does not match what the reagent expects. | Confirm the sample type printed on the box before the draw, not after. |
| Two part-used boxes open at once | Different lots interleaved through one curve, adding scatter that reads as biological variation. | Finish one box before opening the next, and note the lot against the cycle. |
What to check before you open a box
- The market version. Packaging differs by region. If the box does not look like the last one, stop and ask before running it.
- The sample type. Whole blood and serum are different products with similar names.
- The analyte. The platform runs a menu of tests. Progesterone is one of them.
- The expiry date, and how it was stored in transit. A kit that spent a week in a hot vehicle is not the kit that left the warehouse.
- The lot. Write it down against the cycle. If a curve later looks wrong, the lot is the first thing anyone will ask you for.
If you think you have run a mismatched kit
Do not try to correct the numbers arithmetically. There is no factor you can apply, because the relationship between the two curves is not a constant offset — it changes across the range, as the figure above shows.
Run the tri-level quality control that shipped with the analyzer using a kit you trust. If the controls read correctly, the instrument is fine and the affected results should be discarded rather than reinterpreted. Where a breeding decision has already been made on those numbers, treat the day count as unconfirmed and re-establish it from a fresh series.
Frequently asked questions
Will the analyzer warn me if I use the wrong test kit?
No, and that is the whole problem. The instrument reads a signal and applies the conversion curve the kit hands it. If the curve belongs to a different product, the analyzer is doing exactly what it was built to do — and the number on screen is in the right units, inside a plausible range, with nothing to flag it.
Can I correct results from a mismatched kit with a conversion factor?
No. The relationship between two calibration curves is not a constant offset; it changes across the range. Discard the affected results rather than reinterpreting them, run the tri-level quality control with a kit you trust to confirm the instrument itself is fine, and re-establish the day count from a fresh series.
Are whole-blood and serum kits interchangeable?
They are separate products with similar names. A sample prepared for one is not valid on the other, so the sample type printed on the box has to be confirmed before the draw — once blood is in the wrong tube the decision has already been made for you.
How do I know a kit is authorised for my region?
Buy from a channel that can tell you the lot history rather than from a marketplace listing, and treat unfamiliar packaging as a stop sign: kits are released per market and the boxes differ. If a new box does not look like the last one, ask before running it.
Sources
PETlife canine progesterone (cProg) test kit package inserts, whole blood and serum or plasma versions.
Nöthling JO, De Cramer KGM. Comparison of progesterone assay by chemiluminescence or radioimmunoassay for clinical decision-making in canine reproduction.
Wondfo USA. PETlife Pro-DX Immunofluorescence Analyzer, published test menu specifications.
Keep reading
Not sure whether a box is the right one? Call 1 (866) 375-4357 or email vethelp@wondfousa.com with the lot number, Mon–Fri 9:00 AM – 6:00 PM PT.
Product specifications and equipment requirements cited here are those published by the manufacturer at the time of writing; always confirm against the manual and the insert supplied with your kit. This article is educational and is not a substitute for veterinary advice. Threshold values are assay-dependent and are not interchangeable between analyzers. Breeding decisions and result interpretation should be made in consultation with your veterinarian.