For the critically ill, blood sugar that's even slightly higher than normal increases the chance of death, says a study that tracked 216,000 intensive-care patients in veterans hospitals.

The risk was highest in patients suffering heart attacks, strokes or other cardiovascular conditions — and in patients with undiagnosed diabetes, concluded research presented at a meeting Monday of the American Diabetes Association.

Doctors have long known that diabetes both causes other deadly diseases and complicates their treatment. More than 6 million Americans are estimated to have undiagnosed diabetes.

Monday's study suggests all patients in intensive care units should have their blood sugar checked. It tracked the medical records of 216,000 patients admitted to 177 Veterans Affairs ICUs, correlating blood-sugar levels with risk of death for each patient's medical condition.

Even modest hyperglycemia — starting at just one point above normal glucose levels — was associated with mortality, reported researchers with the Cincinnati Veterans Affairs Medical Center.

The higher the level, the worse the outcome, but not for every disease. Some conditions, such as liver failure or chronic obstructive pulmonary disease, were unaffected.

The risk was worst for stroke victims, increasing mortality between 3.4 and 15 times the rate expected.