Listen up, diabetics: invisible bits of fruit on your hands can mess up your finger-prick blood tests, making your blood sugar level look higher than it really is.

That’s because sugars from fruit will stay on your fingers until you wash them with tap water, a new study shows.

Even alcohol swabs don’t solve the problem.

The researchers say that peeling fruit right before you use a blood sugar meter, or eating some juicy fruit with your hands, could lead to an inaccurate reading even if you rub your finger with alcohol first.

Blood sugar meters work by taking a drop of blood from the tip of the finger and testing the sample for how much sugar is in the blood. Many people with diabetes have to use these meters a few times a day to monitor their blood sugar levels and determine how much insulin to take.

Close to 20 million people in the U.S. have diabetes, according to the Centers for Disease Control and Prevention.

For the new study, published in the journal Diabetes Care, Dr. Takahisa Hirose of the Juntendo University Graduate School of Medicine in Tokyo, Japan, and colleagues found ten healthy volunteers. None of the volunteers had diabetes; they all had normal blood sugar levels.

The researchers measured the blood sugar levels of all volunteers under a variety of conditions. First, participants swabbed a finger with alcohol and did the test before handling any fruit, to find their true blood sugar levels.

Next, the volunteers peeled oranges, grapes, or kiwis and checked their blood sugar without cleaning up, after cleaning up with alcohol, and after washing with tap water.

When fruit peeling was followed by hand washing, blood sugar readings were the same as before volunteers had peeled any fruit - generally around 90 milligrams per deciliter (mg/dL), considered in the normal range.

But when volunteers peeled fruit and took a blood sugar reading right away, the levels shown by the blood sugar meter shot up - to about 170 mg/dL on average after peeling an orange, 180 mg/dL after peeling a kiwi, and 360 mg/dL after peeling a grape.