Does keeping fist clenched while blood is drawn cause false readings?

Yes, keeping a fist clenched while blood is drawn can cause false readings. Clenching can lead to increased pressure in the veins, resulting in hemoconcentration, which alters the concentration of certain substances in the blood. This may affect the accuracy of tests, particularly those measuring electrolytes, proteins, and other analytes. It's generally recommended to maintain a relaxed state during blood draws for more accurate results.