Isabel Viña, health expert: "I'm going to tell you a little trick before you get your blood tested."

Healthcare professional Isabel Viña has shared a useful tip on her TikTok profile for those about to undergo blood tests: "I'm going to tell you a little trick: before you get your blood tested, drink a glass of water," she says in her video. This habit, as simple as drinking between 200 and 250 milliliters of water just before the blood test, can make a difference. Although many people go to the lab on a strict fast, the truth is that maintaining a minimum level of hydration improves the reliability of the results without interfering with fasting.

Viña explains that presenting dehydration can cause certain values to appear falsely elevated in blood tests. For example, the hematocrit —which measures the proportion of red blood cells —may show higher than actual values. Other indicators such as sodium , creatinine , or glomerular filtration rate may also be affected. These changes don't necessarily reflect a health problem, but they can lead to misinterpretations or the need to repeat tests .

Drinking water before pumping does not break the fast.

One of the most common questions among patients is whether it's permissible to drink water before a blood test. Viña clarifies that it is: drinking a small glass doesn't break the fast and is, in fact, beneficial. It's not about drinking large amounts, but rather a reasonable amount— about 200 or 250 milliliters —that promotes hydration and can prevent dizziness . It also helps the extraction go more smoothly , especially for people with difficult veins or who tend to get nervous.

Isabel Viña's video has been very well received for its approachable tone and practical content. Through platforms like TikTok, healthcare professionals like her are bringing medical information to the general public . In this case, it's a recommendation based on clinical experience that improves patient preparation and reduces the possibility of errors in results. A gesture as simple as drinking a glass of water can have a positive impact on the quality of a diagnosis .