Limitations of Inhibin B Testing

Posted on May 25, 2005 by Inception Fertility

Some patients might have noticed claims that an Inhibin B blood test can better help determine her egg quality. We at Pacific Fertility Center have examined this topic carefully and have chosen not to incorporate this test as a routine procedure. Currently we use a number of parameters to determine egg quality, or ovarian reserve. For most patients this includes review of: 1. The female partner's age, 2. Results of cycle day 3 FSH and 3. Estradiol (estrogen) testing or 4. A complete clomid challenge test (CCCT), and 5. Ultrasound to determine basal antral follicle count (AFC). With these parameters we can help determine chances of success with each treatment modality. We are constantly looking for ways to better determine ovarian reserve. One proposed adjunct is a blood test for Inhibin B. Inhibin B is a protein secreted by the resting antral follicles in the ovary, and is responsible for inhibiting the secretion of FSH in the early part (follicular phase) of the menstrual cycle. There is also a second inhibin called Inhibin A. This inhibin is secreted by the selected and growing follicle in the second (luteal phase) part of the menstrual cycle. Inhibin B is secreted by the group of small, resting follicles in the ovary and indicates a woman's ovarian reserve. The higher the Inhibin B level, the more ovarian follicles are present in the ovary, the greater the chance of growing a number of follicles with stimulation medications, the greater the chance of achieving a pregnancy. Most studies indicate that an Inhibin B level = 45 pg/ml would indicate adequate ovarian reserve. Inhibin B levels decrease as women age and total follicle numbers decline. Women with very low Inhibin B levels (<20 pg/ml) have such poor ovarian reserve that they have a very high chance of cancellation in an IVF cycle. Inhibin B is a direct measurement of the hormonal dynamics of the ovarian follicles. FSH testing is an indirect measure of ovarian reserve, but the FSH test is readily available at most reference laboratories. Inhibin B testing is more laborious, and few labs offer this test. Additionally, numerous studies have shown that doing an Inhibin B test alone does not provide more accurate information nor better predict one's ovarian reserve, compared to an FSH test alone. Therefore, these 2 limitations have not allowed for the incorporation of routine Inhibin B testing in a fertility evaluation. -- Isabelle Ryan, MD

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