Fasting before getting a blood test of cholesterol levels may be unnecessary, a new Canadian study suggests.

Current guidelines suggest taking blood samples for lipid tests like high-density lipoprotein or HDL cholesterol after fasting for nine to 12 hours, but the requirement isn't always practical for patients.

Fasting for routine blood work may discourage patients from going for the tests and blood labs may have long wait times in the morning that inconvenience people even more.

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Fasting for routine blood work may discourage patients from going for the tests. (Jose Leiva/Sun Journal/Associated Press)

In Monday's issue of the Archives of Internal Medicine, Dr. Davinder Sidhu and Dr. Christopher Naugler of the University of Calgary looked at how long 209,180 people in Calgary fasted and their lipid results. Last year, the city's laboratory service changed its policy allowing samples to be processed regardless of the fasting time.

"We found that fasting time showed little association with lipid subclass levels in a large community-based cohort," the study's authors concluded. "This finding suggests that fasting for routine lipid level determinations is largely unnecessary."

HDL and total cholesterol levels varied less than two per cent with different hours of fasting while triglyceride levels varied by about 20 per cent, the researchers found.

"The elimination of a fasting requirement for lipid determination could also increase patient compliance with testing, which could have particular benefits for patients with diabetes, many of whom have difficulty with prolonged fasting."

The study's authors pointed out they weren't able to consider meal choices before the blood draws. They also weren't able to control  for errors in the self-reported fasting times or the medications that patients were taking.

Proponents of fasting for lipid testing say fasting was used in most studies on predicting cardiovascular risk and in most clinical trials of statins, Dr. Samia Moria of the medicine department at Brigham and Women's Hospital in Boston said in a journal commentary accompanying the study.

The length of fasting wasn't randomly assigned in the study, which included participants who were relatively young with an average age of 53. Moria cautioned the results may not apply to patients at higher risk, such as those with diabetes.

The usefulness of fasting for the tests in terms of greater precision in some measurements needs to be weighed against the challenges and perhaps missed opportunities imposed by requiring fasting and a morning trip to the lab, Moria said.

Moria called for more studies to validate the nonfasting approach before it's universally endorsed, adding it's reasonable to consider in most people coming for a routine clinical visit.

A journal editorial by Dr. Michael Gaziano of the aging division and Brigham and Women's and Harvard suggested that doctors start with a nonfasting lipid profile to assess a patients' risk, decide about starting treatment and then monitor the effects of treatment. But some fasting may be useful to monitor triglyceride levels.

"With this approach, most of our lipid profiles can be obtained in the nonfasting state, increasing convenience for our patients and ourselves and decreasing the burden on the laboratory, with no real adverse effect on clinic decision making," Gaziano concluded.

Gaziano and the study's authors said they had no conflicts of interest. Moria has consulted to Pfizer and Quest Diagnostics and received speaking fees from Abbott, AstraZeneca and the National Lipid Association for educational activities.