Diabetes advocates are the latest group to complain that the way the federal disability tax credit is being applied is unfair.

On Monday CBC News reported that parents of children with a rare disorder called phenylketonuria, or PKU, are frustrated that some families are approved while others are not.

By contrast, according to the Canadian Diabetes Association, most children with Type 1 diabetes do qualify for the federal benefit.

However, once they turn 18, many of them are cut off.

Canadian Diabetes Association spokeswoman Joan King says her group is calling on the federal government to allow all Type 1 diabetics access to the tax credit.

"The fact that they're perceived to be independent of adult support seems to be the clincher but they still have to manage a 24/7 complicated and demanding disease," she said.

The sticking point seems to be a strict requirement that patients spend at least 14 hours per week administering a life-sustaining therapy — in this case, insulin.

King says children are generally approved because the Canada Revenue Agency (CRA) adds up the time spent by both parents and children.

Another problem is that only certain activities count, King says. For example, tracking carbohydrates and managing blood-sugar lows do not.

"There are many activities that aren't permitted. And when we calculate it's a 24-7 complicated disease to manage that the CRA doesn't permit," she said.

The association submitted its recommendation to change the rules to the federal government earlier this year.