False negative blood test - is this possible?

[Ptejhe]

I have a mini mare that I truly think is pregnant. I did the WeeFoal test 38 and it came back positive. So I had our Vet come and do blood work to see if she was pregnant and the results were negative...that she is not pregnant.

But, she is getting much bigger and I was wondering if it's possible to get a false negative blood test?

 

[amysue]

It is possible but more likely the urine test was inaccurate as the 38 day test has a tendancy to be wrong a lot. An inaccurate blood test would be attributed to operator error or compromised sample which is less likely being done by a vet. Have you been feeding her more because you thought she was pregnant? That would attribute to the weight gain.

 

[Minimor]

I don't know about now, but years back it was fairly common to have inaccurate blood test results for pregnancy--I know a number of people (differernt places, different vet clinics) that tested their mares and had results that proved to be inaccurate. That is one reason why we have never bothered to blood test our mares for pregnancy.

 

[targetsmom]

Another possibility is that she WAS pregnant and had an early abortion that went unnoticed....

 

[happy appy]

I did the urine test and the blood test last year and both told me that Sugar wasn't in foal. She didn't even get big until the last couple of months. Then she had this little guy!

 

[ohmt]

My vet will not do the blood test for me anymore because it has to be done at the exact right time during the possible pregnancy to give an accurate reading and I do a lot of pasture breeding so it's just not possible to know how far along they should be. I believe it was closer to the middle of the pregnancy that it would be accurate? I could be wrong....it's been a while since I tried to use it! Had great success with the wee foal tests.

 

[Matt73]

I've had 100% accurate tests with both the 38 and 120 wee foal tests...

 

[Little Wolf Ranch]

My vet won't even offer the blood tests for pregnancy because he says they are so unreliable.

 

[drmatthewtaylor]

'Blood test' is too vague of a description to know what was done.

Many used progesterone to diagnose pregnancy, but progesterone fluctuates daily and can have a high degree of inaccuracy.

Most today use Estrone Sulfate which is about 95% accurate, but needs to be done after 100 days of pregnancy and accuracy might fluctuate between labs.

Blood tests tend to have a poorer sensitivity (false negatives) than palpation and ultrasound. Some of the false negatives can be as high as 53%, while still having a high accuracy. This might be confusing as specificity, sensitivity, and accuracy are statistical terms with exact meanings that are slightly different than what the casual observer might think.

Most skilled Veterinary Ultrasonographers would consider their accuracy to be >95%, plus ultrasounds provide more information than just +/- pregnancy.

Most Veterinarians only consider blood tests when palpation and ultrasound are impossible to do. For my part, I think they are always possible and nearly never do blood tests.

Dr. Taylor