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Pre-Medicare retirees need to lock in new individual health insurance under the Affordable Care Act by Dec. 15 for coverage starting at the beginning of the year, but rushing into a new plan could be costly, experts say.

The full open-enrollment period for the health insurance marketplace runs through Feb. 15. But current participants who haven’t changed their coverage will be re-enrolled in their existing plans Dec. 15, though they can make changes to that coverage until Feb. 15, said Caroline Pearson, vice president of Avalere Health, a health-care consulting firm. And to have new coverage take effect at the beginning of 2015, consumers need to choose new plans by Dec. 15.

Though the clock is ticking, blindly staying with your old plan or choosing one based on monthly premiums alone is a recipe for a costly mistake, Pearson said.

“Honestly, predicting which plan will be the cheapest is incredibly challenging,” she said, not only because consumers in any given market could be choosing from hundreds of possible plans but also because they can’t know with certainty what their medical needs will be in advance.

That said, experts recommend taking a few concrete steps to getting the best plan:

Start with a primer. If you’re just getting started, the Kaiser Family Foundation offers an explainer for 2015 plans and a calculator that quickly lets consumers know if they may qualify for premium subsidies. Check out the calculator here.

According to Kaiser, 35 states will see increases in the number of insurers offering plans next year, while the number will decline in two states.

Premium changes in 16 cities Kaiser studied will range from increases of nearly 9 percent to decreases of nearly 16 percent.

If you’re certain you won’t qualify for premium subsidies, check out off-exchange individual policies in your area. Many insurers offer virtually identical plans whether they are on or off the exchange, but there can be differences, particularly in the breadth of provider networks, said Cynthia Cox, a senior policy analyst for Kaiser.

Narrow the field. Rather than exhaustively comparing pricing on all the plans available to you, narrow the field a bit by checking out which plans work with your physicians. Whether it’s a beloved internist you’ve seen for years, or you simply appreciate a doc-in-the-box at a corner near home, you’ll save yourself some aggravation by starting with health-care providers you actually want to see, experts said.

Run the numbers. Once you’ve selected insurers providing access to your health-care providers, the next step is deciding which of the “metals” — plans categorized by bronze, silver, gold or platinum — makes sense for you.

“Plans are required to cover a minimum set of services and that will be common across the metals,” said Cox. “The difference is in how much you pay out of pocket. If you have significant savings and can afford a higher deductible and are healthy, you may be better off in a bronze plan,” she said. In effect, a consumer could self-insure knowing that he or she could handle paying up to the out-of-pocket maximum in a worst-case scenario.

You might not want to go with the lowest-cost monthly premiums in at least two situations. First, if your annual income is below 250 percent of the poverty line — about $39,000 for a household of two — then you could qualify not only for premium subsidies but also for help with out of pocket costs. To get help with the out-of-pocket items, you must be in a silver plan.

(If you wouldn’t qualify for out-of-pocket help but do qualify for premium subsidies, you aren’t required to choose a silver plan.)

Or if you’re doing some post-retirement freelance work, you might opt for richer premium categories and less out-of-pocket liability because the premiums can be tax deductible as business expenses. So talk to your tax preparer about the best strategy.

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