Blood tests are a normal part of a good health regimen and are often included in physical exams for adults of all ages. Anyone who has scheduled a preventative care visit that includes a standard panel of blood tests has likely heard the same warning: don’t eat the morning before your test. But why is this the standard in blood testing? What does fasting do to improve the quality of results?
Even though the idea of fasting for any amount of time can seem daunting, particularly for those who are used to eating first thing in the morning, fasting before blood tests can positively influence results. This is why you should fast before blood tests, and what consequences failing to do so can cause.
Diet and the Effects on Blood
Fasting is often recommended before a blood panel due to the effects food intake can have on the blood. Many substances we eat, from caffeine to cholesterol, are broken down and absorbed into the bloodstream, which can affect things like blood sugar, for example – hence the stereotype of diabetics carrying around candy bars in case of a sudden blood sugar crash. Eating too close to when you have blood drawn can result in a reading based on your diet, not your body.
In healthy humans, misread results based on diet are unlikely to cause serious issues. However, failing to fast can lead to misdiagnosis or a failure to diagnose new or worsening issues if any are present, compromising overall health and the efficacy of treatments. Eating before a blood test can impact the results to tests that screen for things like:
- High cholesterol
- Liver disease
Not all blood tests require fasting to be accurate, but it’s not uncommon for doctors to suggest fasting before all blood panels due to the broad nature of standard tests.
Blood Tests That Require Fasting
Blood tests aren’t all made equal, but numerous forms of blood panels are improved by cessation of food intake within the hours prior to a blood test. The most common examples of these screenings are as follows.
Blood Glucose Tests
Fasting blood glucose tests are among the most common tools used to diagnose diabetes. A way to measure the levels of sugar in the blood, a blood glucose test relies on unaffected readings to come to a true diagnosis without the influence of diet. While the time period required for fasting varies from one test to another, fasting for at least eight hours is encouraged before a blood glucose test.
An organic molecule found throughout the cells of the body, cholesterol can be indicative of numerous health consequences when found in high concentrations. Due to the fatty nature of cholesterol, any food consumed can change readings, obscuring test results. As such, those undergoing tests are encouraged to refrain from eating in the nine to 12 hours following a procedure.
Cholesterol tests come in several forms, including:
- HDL cholesterol, or good cholesterol
- LDL cholesterol, or bad cholesterol
High cholesterol can lead to numerous conditions, including stroke, heart disease, high blood pressure, and diabetes. Testing after eating can mask these conditions, leading to an inaccurate diagnosis.
Gamma-Glutamyl Transferase Test
Gamma-glutamyl transferase tests are an essential part of evaluating liver disease. As an enzyme that helps the liver properly function, abnormal levels can help identify the early stages of potentially serious issues. Food generally doesn’t affect gamma-glutamyl transferase levels but smoking cigarettes and alcohol can. As such, doctors generally recommend fasting.
Iron can be tested in several different ways but blood tests are the most common. As a blood panel, iron tests examine levels of iron in the blood in order to look for conditions indicated by a lack of iron, like anemia.
While food in general can’t influence a blood iron test, the wrong kinds of foods can. Iron is a common supplement in food and eating too much of it can negatively influence the results gained in an iron test. To prevent inaccurate levels of iron, those undergoing tests are encouraged to forgo iron intake in the hours directly preceding a test.
In addition to the major tests that require fasting, several other blood tests can be easier to read or more accurate after an appropriate fast. These include:
- Standard metabolic tests, including blood sugar, kidney function, and electrolyte balance
- Rental function tests
- Vitamin B12 tests
Tips and Tricks for Fasting
For breakfast eaters or coffee drinkers, abstaining from intake can be a challenging proposition. These tips can make the prospect of fasting easier to manage until your blood draw is complete.
- Drink plenty of water. Water can help you feel fuller, minimizing feelings of hunger
- Determine the latest time possible to eat and be sure to do so. For example, if you have to fast for 10 hours and your test is at 10:00 AM, the latest you can eat is midnight.
- Don’t neglect any regular medication, even those intended to be taken with food.
- If you are pregnant, be sure to consult your doctor before fasting.
Fasting can be an important part of ensuring the results of blood panels are accurate. This is particularly true when testing for conditions like high cholesterol or diabetes; untimely eating can alter test results, leading to incorrect diagnoses and potentially missed health problems in need of treatment. Fasting is rarely fun, but when it comes to blood panels, taking this step can go a long way in ensuring an accurate outcome.