When I worked as a scribe prior to starting medical school, it was commonplace for patients to have fasting labs. I always felt terrible for the patients we saw late in the afternoon that had somehow fasted all day. For many other patients, there was the challenge of finding a time when they could return to have fasting labs drawn.
However, I have still observed instances when patients need to have fasting labs. We can look at an example case to better understand when and why patients do and do not need to fast prior to having their lipids checked.
A 57-year-old woman presents for an annual wellness visit. She has been healthy this past year with no new concerns. Her blood pressure has been well controlled, and she continues on a statin for hyperlipidemia. She is due for annual labs. She ate breakfast this morning. Which of the following do you recommend?
A. Obtain lipids with her other blood work now.
B. Have her return tomorrow to obtain fasting labs.
In this situation, A is the correct answer. The patient is due for routine screening labs and there are no current indications that fasting labs are necessary.
Studies of fasting vs. nonfasting lipids
Sidhu and Naugler performed a cross-sectional analysis comparing lipid values at fasting intervals of 1 hour to 16 hours.1 They found the mean total cholesterol and HDL cholesterol values differed by greater than 2%. For LDL cholesterol, the values differed by less than 10% and triglycerides values differed by less than 20%. With this information, the researchers concluded fasting for routine lipids is generally unnecessary.
Mora and colleagues performed a post hoc prospective follow-up of a randomized control
trial to assess if nonfasting lipid measurements could cause misclassification of cardiovascular risk assessment.2 Based on 8,270 participants, coronary events associated with fasting vs. nonfasting lipid values were similar when adjusted hazard ratios were compared. They also found an agreement of 94.8% when classifying participants into ASCVD risk categories for fasting and nonfasting lipid values. These outcomes led them to support the use of nonfasting lipid labs for routine cardiovascular risk assessment.
Rahman and colleagues performed a systematic review and found the use of nonfasting lipid values can reliably determine statin management in most situations.3 Circumstances where fasting labs should be used are if patients have a genetic dyslipidemia, if patients have severe hypertriglyceridemia (greater than 500 mg/dL), and if patients have pancreatitis. Triglyceride values fluctuate the most between the fasting and nonfasting state as seen above from Sidhu and Naugler. This could impact triglyceride disorder management and the accuracy of LDL cholesterol estimation (calculated by the Friedewald equation: LDL cholesterol = total cholesterol – HDL cholesterol – triglycerides/5 in mg/dL).3