The rapid decline of U.S. oil prices will test the claim of fracking companies that they can now prosper at $50 a barrel or less, a price level they have found challenging in the past.

For years, the companies behind the U.S. oil and gas boom, including Noble Energy Inc. and Whiting Petroleum Corp. , have promised shareholders that they have thousands of prospective wells that they can drill profitably even at $40 a barrel. Some have even said they can generate returns on investment of 30%.

But most shale drillers haven’t made much, if any, money at those prices. From 2012 to 2017, the 30 biggest shale producers lost more than $50 billion. Last year, when oil prices averaged about $50 a barrel, the group as a whole was barely in the black, with profits of about $1.7 billion, or roughly 1.3% of revenue, according to FactSet.

U.S. oil prices traded below $50 last week and closed Monday at $52.95.

The disconnect between the figures cited by companies and their corporate returns lies in the widespread use of a metric called a break-even, often defined as the selling price frackers say they need to generate a small profit on individual wells or projects. While the figure can be quite low for some companies in certain hot spots, it can be a misleading measure of their overall profitability in periods of lower prices.