When measuring progesterone for breeding management, there are many cases when sending blood to an outside reference laboratory (with next-day results) is adequate. However, in situations when timing needs to be exact, such as with frozen semen, individuals with less-than-ideal fertility, or logistically-challenged breedings, having the results the same day can make a big difference. Clinics with the ability to analyze progesterone levels in-house can often return results in as little as an hour, maximizing the time available to make management decisions.  


In the past few decades, and especially in the past few years, there has been a rise in the availability of in-house progesterone analyzers. In particular, there was a big buzz created earlier this year (2019), when Idexx (one of the major lab companies) added the ability to measure progesterone to their Catalyst II analyzer. This analyzer is incredibly common in small animal clinics and essentially overnight gave thousands of vet clinics the ability to measure progesterone in-house. This has raised awareness of progesterone testing, but has also raised an important question: Can we trust the numbers from an in-house analyzer or are we better off sending it out to a reference lab? 


There’s more than one way to measure progesterone, and different analyzers use different methods. The most accurate way to measure progesterone is with mass spectrometry, but this method is effectively only used in research due to the cost and equipment required. Even when we “send out” a blood sample to a reference laboratory, they do NOT typically use mass spectrometry to measure the amount of progesterone. Instead, most reference clinics are using an Immulite® analyzer, which “tags” progesterone with a luminescent (glowing) chemical and then measures the amount of luminescence (glow) in the sample. Other analyzers use other methods, but in essentially all cases, the analyzer is “doing something” to the progesterone and then looking for evidence of the thing it did. 


Reference lab values from an Immulite® are imperfect, as are all results, but they are the most commonly used. When we talk about progesterone numbers (i.e. LH surge at 2 ng/mL, ovulation between 5 ng/mL and 10 ng/mL), by default we are referring to values as they are measured using a reference lab Immulite®. When veterinarians get a new in-house analyzer, they will spend several weeks or months running samples in duplicate to track how their analyzer compares to the reference lab. This means they will divide one blood sample into two, then run one on their in-house analyzer and send one out to their reference lab. 


When you call a vet clinic to ask about their in-house progesterone capability, the clinic should be able to report something to the effect of, “The in-house analyzer we use is _______ and our machine typically runs _____ng/mL higher/lower than the reference lab.” It is NOT a bad sign for a clinic to say that their numbers run higher or lower than the reference lab. In fact, it’s a sign of a conscientious clinic that has carefully tested their equipment. Very accurate progesterone timing can still be accomplished with a machine that runs high or runs low, as long as the team using the machine knows how that individual machine runs. Every machine has its own bias, and for the most accurate progesterone tracking, every effort should be made to track a cycle with the same analyzer from start to finish. 


Human error is often the largest source of inaccuracy in progesterone measurements. The in-house analyzers that are recognized as the most accurate – Immulite®, Tosah® , and MiniVIDAS® are also the ones that rely the least on human involvement. Other models often rely on a human operator pipetting tiny amounts of fluids into tiny wells. This is a skill that can be learned, but I can state from personal experience that it takes a lot of practice and is very easy to mess up. 


The Idexx Catalyst also minimizes human involvement. Using the logic above, it should therefore be one of the more accurate analyzers. In the interest of honesty and transparency, I will share that I do not have personal experience with using the Catalyst for progesterone measurement. I have heard some colleagues report great success, including with frozen semen breedings, and others who report wide variation and inability to adequately track the progesterone rise. I’m looking forward to having the ability to test it out myself (hopefully in the next 6 months!). 


Here are the two common objections I’ve heard from colleagues and my response as to why these objections don’t bother me (at least not at this time): 


  • The Catalyst II is not giving me the same values as my reference lab/the machine I was using before


In validating their test, Idexx compared results from their analyzer to results from both mass spectrometry and to the Immulite®. Their results were more closely correlated to the mass spectrometry results than the Immulite® results, which was their goal since mass spectrometry is now considered the goal standard. Idexx openly acknowledges that the results from the Catalyst will not necessarily match results from a reference lab. The main reason this objection doesn’t bother me is for all the reasons explained in this article – every machine is individual. A Tosah® may have slightly different values than a MiniVIDAS®, which may have slightly different values than the other MiniVIDAS® down the street and they might all have different values than the Immulite® used at the reference lab. The important factor is for a clinic to intimately understand their own analyzer and make breeding decisions accordingly. 


  • The Catalyst II is not validated about 10 ng/mL 


This objection does not bother me for reasons described in last week’s article. In general, ovulation will occur somewhere between 5 ng/mL and 10 ng/mL. Once a progesterone reading is over 10 ng/mL, I DO want to see the number continue to rise so I feel confident that ovulation has occurred, but the specific number is not important because there is no “magic number” at which point breeding is ideal. 


What should you do about all this? My suggestion is to talk to your vet clinic. They should be able to tell you which analyzer they’re using, as well as the steps they took to know how their machine compares to a reference lab. Ideally, they’re also tracking pregnancy success rates and using that information to inform their breeding management decisions. Of course, if you’ve been having great breeding success with their system, then you already know they’re on the right track!


I hope this article helped to clarify some of the mystery surrounding in-house progesterone analyzers and helped you to know which questions to ask at your next vet visit. Please let me know if you have any questions in the comments below, and I’d always love to hear your success stories! 


Yours in healthy, responsibly-bred puppies,


Dr. Kristina 

Categories: Breeding


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