The U.S. Supreme Court’s ruling that upheld health care reform generally proved a positive for the stocks of L.A.-area health care companies.

In the four trading days since the June 28 decision upholding the Patient Protection and Affordable Care Act was announced, the stock of only one local company, commercial health insurer Health Net Inc. of Woodland Hills, closed down.

“Our theme for the day was ‘uncertainty removed,’ ” said analyst Judson Clark, who covers Thousand Oaks biotech Amgen Inc. for Edward Jones Co. in Des Peres, Mo. “(Health care sector) executives can get back to running their businesses and meeting the requirements of the new law.”

Health care reform will increase the number of insured Americans but also potentially lower profit margins for many companies. Apart from that, it brings a mixed bag of benefits and challenges for the industry.

Among local companies, Medicare managed care provider Molina Healthcare Inc. of Long Beach was the biggest gainer the day the high court decision came down, jumping 8.6 percent June 28, and closed the week ended July 3 up 5.4 percent. The S&P 500 rose 4.1 percent for the week.

On the other end of the spectrum, Health Net fell 2.3 percent when the ruling came down and finished the week down 2.9 percent.

Analyst Thomas Carroll, who covers Molina and Health Net for Stifel Nicolaus & Co. Inc. in Baltimore, said the market’s reaction illustrates which companies investors believe will have an edge as millions of people enter the health insurance market because of the individual mandate and expansion of Medicare rolls.

“Molina is a Medicaid managed care company that saw its potential market size just get bigger,” Carrroll said. “Health Net, on the other hand, is more vulnerable because it’s a smaller regional insurer in a market where the government will become a larger customer but pays at lower margins. It’s a market where larger companies will have an advantage.”

Carroll said that the pressure on Health Net and other insurers with significant L.A. operations, such as Anthem Blue Cross operator Wellpoint Inc. of Indianapolis was most likely an overreaction by investors who assumed that the Supreme Court would strike down or cripple health care reform.

“Commercial insurers have been pretty weak most of this year, but they gained traction in the three weeks leading up to the Supreme Court ruling,” Carroll said. “So when the individual mandate was upheld, the stocks traded off.”

The stocks of three local drug and medical device makers were either up or down by roughly 1 percent the day of the ruling, but all three went on to end the week with strong gains.

Amgen, which is seen as benefiting from a larger insured population, closed up 5.2 percent for the week. Staar Surgical Co., a Monrovia maker of ophthalmic products, and Iris International Inc., a Chatsworth developer of automated urinalysis testing equipment, gained 11 percent and 14 percent, respectively.

Analyst Christopher Cooley at Stephens Inc. in Little Rock, Ark., found the gains among device makers somewhat surprising, because the ruling makes it unlikely that an industry challenge to a coming tax on medical devices will succeed.

Iris makes products that enable diagnostic laboratories to process patient urine and blood samples at lower cost with fewer employees. The company should benefit from an environment in which hospitals and other providers will be rewarded for greater efficiency.

IPC the Hospitalist Co. Inc. gained nearly 5 percent for the week. The North Hollywood operator of hospital-based physician practices around the country is expected to benefit from the law as the expected drop in uninsured patients takes financial pressure off hospitals. Radnet Inc., a West L.A.-based operator of diagnostic imaging clinics, is expected to see its business grow thanks to the increased number of insured patients.

The region’s three real estate investment trusts specializing in health care properties, including Alexandria Real Estate Equities Inc. of Pasadena, all gained the day of the ruling and continued to rise over the next several trading days. HCP Inc. of Long Beach and LTC Properties Inc. of Westlake Village both own senior care facilities that stand to benefit from increased funding to care for poor seniors.