Follicular sensitivity index (FSI): a novel tool to predict clinical pregnancy rate in IVF/ICSI cycles

This study aims to introduce a new tool (the Follicular Sensitivity Index; FSI) for objective assessment of follicular responsiveness to exogenous gonadotropins and to evaluate its ability to predict the clinical pregnancy rate in women with unexplained infertility or tubal factor undergoing IVF/ICSI.

FSI was calculated as preovulatory follicle count (PFC) × 100,000/[antral follicle count (AFC) × total received FSH doses]. One thousand women were included and were divided according to the FSI tertile values into three groups. The primary outcome was clinical pregnancy defined by the presence of an intrauterine gestational sac 5 weeks after embryo transfer.

There was progressive increase in the clinical pregnancy rate from the low to the high FSI groups (0.27 ± 0.4 vs 0.4 ± 0.4 and 0.58 ± 0.4; p < 0.001). Receiver operator curves showed that FSI had a greater area under the curve than those of the AFC, PFC, and the FSH dose (0.638 vs 0.509, 0.538, and 0.589 respectively). Multivariate logistic regression analysis showed that the correlation between FSI and pregnancy was independent of potential confounding factors like age and body mass index (p < 0.001).

FSI can predict the clinical pregnancy rate in women with unexplained infertility or tubal factor undergoing IVF/ICSI using GnRH agonist protocol. Higher FSI values had significantly higher oocyte yield and fertilization and clinical pregnancy rates. Wider implications of these findings include the potential use of FSI to define absolute criteria of poor/good ovarian response in IVF/ICSI cycles, guide future IVF cycle management for the same couples, and guide cycle cancelation criteria for poor ovarian response.