This was an older test used to check ovarian reserve. The higher your Inhibin level, the better your ovarian reserve. Levels of Inhibin B decrease with age, as your ovarian reserve gets depleted. The Day 3 Inhibin B blood test is performed on Day 3 of your menstrual cycle.

The Day 3 Inhibin B test can help to determine your ovarian reserve

What is Inhibin B?

Inhibin B is a protein hormone produced by the granulosa cells in the ovarian follicles. As the name suggests, its role is to inhibit the production of FSH ( follicle stimulating hormone), which is produced by the pituitary and stimulates follicular growth. Because Inhibin B is produced directly by ovarian follicles, the amount of Inhibin B in your blood directly correlates with the number of eggs left in your ovaries. The higher your inhibin level, the better your ovarian reserve. Levels of Inhibin B decrease with age, as your ovarian reserve gets depleted.

What is the Day 3 Inhibin B Test?

The Day 3 Inhibin B test is a simple blood test that measures the amount of Inhibin B in your blood. It is performed on Day 3 of your menstrual cycle.

The Results
Typically, a normal result is 45 - 200 pg/mL. An abnormal result is anything below 45 pg/mL. This suggests poor ovarian reserve.

Inhibin levels can be used for

1. Evaluating Fertility Potential and ovarian response in IVF - Serum inhibin levels correlate with the number of early antral follicles. This makes is useful for prediciting your ovarian response in an IVF cycle. Women with low inhibin levels are more likely to be poor ovarian responders.
2. Measuring Ovarian Aging - Diminished ovarian reserve, is signaled by reduced baseline serum inhibin concentrations. Women with poor ovarian reserve who have entered the oopause have low levels of inhibin.

The other tests to check ovarian reserve include checking your AMH levels.

However, these are new and expensive tests, and is still not easily available. This is why the gold standard for ovarian reserve testing is still the Day 3 FSH level.