Kolkata: AU Small Finance Bank reported a 53% surge in second-quarter net profit to Rs 858 crore from Rs 561 crore in the year-ago period, helped by business expansion and an improvement in asset quality.The bank's net interest margin—a key profit metric—rose to 5.8% for the quarter from 5.5% in the year-ago period.AU's gross non-performing assets ratio fell 42 basis points (bps) to 1.99% at the end of September.