With cases of diabetes growing each year, many adults are getting caught in a potentially dangerous situation: they are diagnosed with Type 2 diabetes when they actually have Type 1 diabetes, a substantially different condition.

Both types of diabetes make it difficult for patients to control blood-sugar levels, which can lead to complications that include blindness, kidney failure and death. But Type 1 and Type 2 require different forms of treatment.

Alissa Kaplan Michaels, who has Type 1 diabetes, lived for 3½ years with the wrong diagnosis. The New York public-relations consultant says she complained to her doctor in 2008 of blurry vision and was told she had Type 2 diabetes after a blood test showed high sugar levels. She changed her diet and exercised more, but her blood-sugar levels kept rising. She started taking several oral diabetes medications. She stopped eating bread and pasta. She changed doctors—three times. And she still felt terrible.

Last fall, a covering doctor at her endocrinologist's practice started asking about her health history, childhood weight patterns, her recent struggles with her blood sugar and family history of Type 2 diabetes, of which there was none. That day, Ms. Kaplan Michaels got a new diagnosis. She didn't have Type 2 diabetes, she had Type 1.

Ms. Kaplan Michaels, 44 years old, immediately dropped the oral medications that had upset her stomach. Instead, she increased her daily insulin injections. She also resumed eating carbohydrates. Within weeks, her energy was back. "At first I was relieved and then I was very angry," she says. "Nobody should have to go to four doctors to get a diagnosis for something that isn't that difficult to diagnose."