There is a question mark over the quality of the major banks' earnings, following the release of their 2011/12 financial reports, with some analysts suggesting that a substantial amount of revenue growth came from sources that are volatile and unsustainable.The net interest income of the Big Four banks increased 3.8 per cent in the 2011/12 year. Non-interest income, which includes fees and commissions from insurance and wealth management, and sales of foreign exchange, risk management and trade services, increased by 8.9 per cent.Total interest income was $51 billion, compared with $22.8 billion of non-interest income.These results show a shift in focus away from the lending part of banks' businesses, where demand is weak, to other parts of the business.One source of non-interest income is markets income. This includes risk management, interest rate products, foreign exchange, trade finance, cash management and treasury.National Australia Bank reported a 36 per cent increase in markets' income, Westpac a 20 per cent increase and ANZ a 14 per cent increase. Only Commonwealth Bank did not rely on higher markets' income to boost overall income; it reported a fall in markets' income.Macquarie Research issued a note on NAB's result, which said: "Risk income from FICC (fixed income, currencies and commodities) and treasury increased significantly. We believe this is unlikely to be sustainable."Macquarie said: "The increase in non-interest income was driven by volatile trading profits not likely to be sustainable in 2012/13."Macquarie was also critical of the high level of trading revenue revealed in Westpac's results.However, the head of KPMG Financial Services, Michelle Hinchliffe, said it was important to recognise that not all markets' income was volatile. Hinchliffe said: "Trade finance, cash management, risk management, interest rate products and fixed income can be classified as low-volatility sales activities. "The high volatility activity is where they are taking a view on markets in their treasury operations. "Our view is that customers' sales make up the majority of markets' revenue for ANZ and Westpac, and about half for NAB."PwC also commented on the results. It said: "Trading income was a real positive. Continued demand from customers for risk mitigation products has been a noticeable positive, as has the growth in trading income from offshore expansion initiatives."