From the Guidelines
Patients should fast for at least 8 hours before bloodwork, and drinking water is allowed and encouraged during this period. This recommendation is based on the most recent guidelines for laboratory analysis in the diagnosis and management of diabetes mellitus, which suggest that blood for fasting plasma glucose analysis should be drawn in the morning after the subject has fasted overnight (at least 8 h) 1.
Fasting Requirements
The purpose of fasting is to ensure accurate results by preventing food and beverages from temporarily altering blood components like glucose, lipids, and certain enzymes that are commonly measured in blood tests.
- Fasting for at least 8 hours is generally recommended for most routine blood tests, typically overnight.
- Water consumption is allowed and encouraged during the fasting period to maintain proper hydration.
- Patients should avoid coffee, tea, juice, soda, alcohol, and any other beverages besides plain water during the fasting period.
Special Considerations
Some specific tests may have different requirements, such as:
- Certain hormone tests or specialized metabolic panels might require complete fasting, including water.
- Basic blood counts may not require fasting at all. If taking medications, patients should ask their healthcare provider whether to take them before the test, as some medications can affect test results 1.
Importance of Fasting
Fasting is crucial for accurate diagnosis and management of diabetes, as it allows for the measurement of fasting plasma glucose levels, which is a key diagnostic criterion for diabetes 1. The American Diabetes Association recommends that fasting plasma glucose levels be measured in venous plasma after an overnight fast of at least 8 hours, with a value ≥126 mg/dL (7.0 mmol/L) diagnostic of diabetes 1.
From the Research
Fasting Duration Before Bloodwork
- The duration of fasting before bloodwork is typically recommended to be around 12 hours, as seen in the study 2.
- This study found that patients who were verbally instructed to fast for 12 hours prior to their appointment had a higher adherence to fasting requirements compared to those who received no instructions.
- However, the study also found that the biochemical analyses appeared unaffected by fasting status, suggesting that the impact of fasting on bloodwork results may be minimal.
Drinking Water Before Bloodwork
- There is no clear evidence to suggest that drinking water before bloodwork has a significant impact on the results.
- The studies provided do not specifically address the issue of drinking water before bloodwork, but it is generally recommended to follow the instructions provided by the healthcare provider or laboratory conducting the test.
Biochemical Parameters Affected by Fasting
- The study 2 found that median concentrations for potassium and total bilirubin were statistically, but not clinically, significantly different between the intervention and control groups.
- The study also found that triglyceride concentration and lipaemia were not significantly different between the two groups, suggesting that fasting status may not have a significant impact on these biochemical parameters.
- However, it is essential to note that the study 3 found that a very-low-carbohydrate diet achieved greater improvements in the lipid profile, blood glucose stability, and reductions in diabetes medication requirements, suggesting that dietary factors can have a significant impact on biochemical parameters.