How many multiple sclerosis patients are in the U.S.? No one knows.

Jul 30, 2015, 2:50pm EDT

The multiple sclerosis market is a crucial one to the local biotech industry, with two of Cambridge’s biggest drugmakers relying heavily on sales of drugs to treat the disease for future growth. But despite all the attention on selling drugs for the disease, the data on how many patients actually suffer from it are surprisingly fuzzy.

Biogen (Nasdaq: BIIB) and Genzyme, a subsidiary of Sanofi(NYSE: SNY), both reported financial reports for the second quearter in the past week which focused on sales of their MS drugs. Both drugs happen to be pills, and both were launched in the last two to three years, and both cost about $66,000 in the U.S. (although no one actually pays that price due to hefty discounts). In Biogen’s case, Tecfidera sales of $883 million from April through June represented a 26 percent year-over-year increase but didn’t meet expectations, spurring the company to lower its full-year guidance. For Genzyme, sales of Aubagio were $224 million, which was up 80 percent over the same time last year, growth which seemed to surprise executives.

There’s a general recognition that growing competition for MS drugs is putting pressure on costs, which have risen substantially in recent years. Estimates of the total size of the market in coming years range from $18 billion to $21 billion a year worldwide. The vast majority of that revenue is expected to come from the U.S., where prices are four to five times higher than in Europe.

But nobody seems to know exactly how many MS patients there really are. Market reports give a range of 350,000 to 500,000 patients in the U.S. — a huge range, which could affect the market substantially. Bruce Bebo, executive vice president of research at the National MS Society, says the organization’s estimate of about 400,000 patients in the U.S. is based on epidemiology-based population estimated developed in the 1970s. While the MS Society has updated its numbers to reflect new census data, the estimates of the percentage of the population that has the disease are based on formulas developed 50 years ago, and need to be revisited.

The society has been pushing for some kind of national registry for years, and Bebo says the pending 21st Century Cures Act now before Congress contains provisions for a national neurological diseases surveillance system that would help. Meanwhile, the MS Society is pursuing other ways to refresh the numbers based on a more scientific method than the updates.

But Bebo says for now, there is a lot of speculation. Many people think the number of patients is growing, and Bebo said there are “credible studies” in Canada and Europe that suggest that number of new cases is rising, but those studies are “somewhat controversial”, he said. Many believe the reason is likely the longer lifespan of patients due to better treatment, but no one really knows.

“There seems to be a consensus that there’s a increase in the prevalence of multiple sclerosis,” he said.