Most of the biggest banks in Latin America saw their net interest margins fall in the third quarter from a year earlier as economic growth concerns led central banks in the region to reduce borrowing costs.

A bank's net interest margin, or NIM, is a key profitability metric that measures the difference between interest income generated and the amount of interest paid, relative to the amount of its interest-earning assets.

In a sample of 15 top banks analyzed by S&P Global Market Intelligence, 11 booked lower NIMs year over year while the remaining four posted higher ratios.

Ratios for the six Brazilian and Mexican banks in the group all fell, with NIMs at Banco Santander (Brasil) SA and Itaú Unibanco Holding SA dipping into negative territory. Monetary authorities in both countries, the two largest economies in Latin America, announced multiple rate cuts in the third quarter.

Banco BBVA Argentina SA, meanwhile, ended the period with a NIM of 21.41%, up from 13.97% a year earlier, as soaring inflation and market volatility in Argentina forced the country's central bank to impose a 58% floor for its benchmark Leliq rate through most of the third quarter.