have had to starve themselves for eight to twelve hours prior to coming in to the office for annual labs. But does this make any sense? As patients know, I urge plenty of skepticism when it comes to recommendations that a doctor might make. Here, I will defend our strategy for doing fasting labs, then I will shoot that defense full of holes, and finally I will present new suggestions for patients regarding their annual labs.
But first, let’s ask a bigger question. Is the whole process of annual physicals and blood tests of any merit, or is this just another money making scam foisted on the American public by physicians looking to fill their appointment slots? Well, the annual exam probably does actually have at least some merit. First, if you’re on a long-term medication like Lipitor or Prozac, then there are certain legal requirements that we have an actual physician-patient relationship, and the requirement can be met by seeing you once per year at least. But for patients on no medications at all, even the most curmudgeonly and stingy of health care experts would agree that screening for high blood pressure, giving flu shots and other needed immunizations, checking to see if patients are depressed, helping folks quit smoking, and screening for diabetes all have value. The interested reader is referred to the 339 page document out of Johns Hopkins titled “Value of the Periodic Health Evaluation,” which is also an effective treatment for insomnia.
Labs. Fasting or Not?
The role of extensive annual blood work, however, is more controversial. In our practice, since we’re going to be drawing blood anyway to screen for some very basic things such as anemia and diabetes, we do run about 50 different tests, looking at the function of the kidneys, liver, thyroid, bone marrow, and screening for abnormal cholesterol, vitamin D deficiency, and several other medical problems. The purist would argue that we run too many tests but honestly, the incremental cost is very small and the risk to the patient in the hands of a reasonably skilled clinician interpreting the results is low.
But what about the whole fasting angle? Well, in the fasted state, we get a more reliable look at blood sugar, which is the main way we screen for diabetes. Also, triglycerides go up after eating and trying to check patients for an elevation in the triglyceride level is much easier if patients are fasting. If patients fast year after year, we have a reproducible baseline with which to compare results, and also, fasting year after year results in a lot of weight loss!
So what, then, is the problem with fasting? Well, for one, patients get hungry. Requiring fasting labs puts in a slight disincentive for individuals to come to the office for their annual exam. Another issue is convenience. If you have to be fasting, most folks would realistically only be able to get labs drawn at 9:00 or 10:00 AM, which does not necessarily fit everyone’s schedule.
The Argument Against Fasting
There are also clinical and scientific arguments against fasting. It turns out that total cholesterol, HDL (good) and LDL (bad) are not really affected much by being in the non-fasted state. In one study, eating lowered the LDL cholesterol by about 7 points at the absolute most. It might seem counter-intuitive that total cholesterol, HDL, and LDL all fall slightly after eating, but this is what the research shows.
It turns out that the variation between fasting and not fasting cholesterol is less than the typical lab-to-lab variation in these values. If you send your blood to two different labs simultaneously, you can easily see a 5% difference in cholesterol values. Only triglycerides go up substantially in the non-fasted state, but even here, there is a nice study showing that non-fasting triglycerides are a better predictor of cardiovascular risk than fasting triglycerides. If you read the National Cholesterol Education Project (NCEP) guidelines for high cholesterol screening and treatment (and these only run to 284 pages), you’ll see that fasting labs are recommended. But one reason is that their approach to measuring LDL (bad) cholesterol is to calculate it from a simple math formula based on the other cholesterol measures. It is called the Friedewald equation, and named after Dr. William T Friedewald. In the old days, measuring LDL (bad) cholesterol was a very complex, expensive lab test and Dr. Friedewald, who went to medical school at Yale and probably had a lot of school loans, published it in 1972 as a cost saving measure. But hey, Dr. Friedewald, the seventies called and they want their formula back, because in our lab we measure LDL cholesterol directly and don’t depend on your equation. This makes our determination of LDL more accurate in the non-fasted state.
This is a good opportunity for me to remind patients that risk factor modification is the most important approach to reducing your risk of heart attack. So of course, quitting smoking, eating well, and getting exercise are very important. However, the biggest risk factors are not cholesterol, but advancing age and male gender. Therefore, patients who are serious about reducing their risk of heart attack should have a sex change (if male) and travel in space close to the speed of light, which will make them age more slowly as compared to the rest of us.
The Diabetes Question
But what about blood sugar? Everybody knows that your blood sugar rises after a meal and falls when you have not eaten. The standard for diagnosing diabetes involves a fasting blood sugar over 125. Still, any non-fasting sugar over 200 on a couple of occasions also makes the diagnosis. More importantly, we screen for diabetes with an additional test, the hemoglobin A1c, abbreviated HgbA1c. This test measures the average blood sugar over the preceding two to three months. Patients often wonder how this is possible. How can a blood test in June tell what a patient’s blood sugar was doing in April and May? Well the answer is that inside each red blood cell is a little man with a clipboard and he keeps track of what you eat and writes down your blood sugar that day on a list.
The hemoglobin inside the red cells
is bathed in your blood
as the cells circulate around the body.
Actually the way it works is that your red blood cells have a lifespan of about four months. The hemoglobin inside the red cells is bathed in your blood as the cells circulate around the body. If hemoglobin is bathed in a high glucose bath, then more glucose molecules end up sticking to the hemoglobin, kind of the way barnacles stick to a dock. The test measures the percentage of glucose stuck to hemoglobin molecules. Normal is under 6% or so and over 7% is certainly in the range of diabetes. Since blood cells are recycled every 120 days, you get an average of newer cells with very little sugar stuck to their hemoglobin and older cells ready to be killed off and eaten up by the spleen that are chock full of sugar and the test takes the overall average. A high percentage indicates high sugar, both fasting and non-fasting, and this is diabetes. One can see that this test does not require fasting on any particular day.
Having dispensed with cholesterol and blood sugar, there still are about 48 other tests in the annual blood panel. We screen for thyroid and liver disease, kidney failure, vitamin D deficiency, abnormalities in body chemistry involving calcium, sodium, and potassium, blood and bone marrow diseases including anemia and leukemia, and other issues as well. None of these tests are substantially affected by fasting.
My new recommendation for patients is that fasting labs are optional. Sure, there may be the occasional person we are treating for very elevated triglycerides where a fasting lab test may be required, and there may be a few patients with “pre-diabetes” or fairly new type II (adult onset) diabetes where we require a fasting glucose. Also, rarely we might end up repeating labs in the fasted state for certain reasons, generally related to cholesterol. But for about 95% of my practice, the convenience and flexibility of doing annual labs in the non-fasting state outweigh any tiny, often theoretical benefit from fasting labs. By removing one more barrier to care, we are likely to see better overall treatment and outcomes for our patients. In fact, since instituting this policy, we’ve already gotten several stragglers in for their annual exams.