So I was very pleased to see that in a recent issue of the Annals of Internal Medicine (17 January 2017 Vol: 166, Issue 2), the Consult Guys state that fasting for measurement of lipids is unnecessary. They cite data from a recent study (1, 2) showing that in both men and women there is virtually no change in the levels of total cholesterol, LDL-cholesterol and HDL-cholesterol. The only test affected by fasting is the triglyceride level which shows about a 20% drop in women and a 25% drop in men with fasting. The Consult Guys do note that it is important for the medical laboratory to actually measure the LDL-cholesterol level. For many years, labs have performed a calculation based on the total cholesterol and HDL-cholesterol levels to provide an LDL-cholesterol result. Commercial labs have been especially notorious for doing this potentially misleading cheap calculation rather than performing an actual measurement. The lipid panels offered by Quest Diagnostics and Labcorp still provide a calculated LDL-cholesterol, not an actually measured LDL-cholesterol. Amazing!
But lipid results are not the only misleading values that can come out of a fasting blood test panel. Glucose results may be misleading in two ways: First, your blood sugar (glucose) might actually be normal but due to fasting it comes back low, which might of course explain why you feel lightheaded or might actually pass out. Second your true blood sugar might be elevated above the current stringent cutoff for abnormal glucose (126 mg/dL) but, because you were fasting, it will be less than that cutoff. Likewise potassium results might be low and misleading from fasting, particularly if it has been hot and you were sweating out a lot of potassium during the night and did not get to eat that banana or orange because you were required to fast. A falsely decreased potassium might trigger your physician to get very nervous and send you to the nearest emergency room to make sure you are not having a heart arrhythmia!
Unfortunately, the practice of requiring fasting for blood tests is extremely entrenched. I have had labs routinely turning away patients who failed to fast until I issued an edict stating that the blood was absolutely to be drawn no matter whether the patient was fasting, and if they were not fasting, all that was required was to put a note to that effect when the results were released.
1. Nordestgaard BG et al. Fasting is not routinely required for determination of a lipid profile. European Heart Journal 37, 1944–1958, 2016.
2. Nordestgaard BG et al. Fasting is not routinely required for determination of a lipid profile. Clinical Chemistry https://doi.org/10.1373/clinchem.2016.258897.