Water loss from the skin is scientifically referred to as transepidermal water loss, abbreviated TEWL.
Q. How is water loss from the skin measured?
TEWL readings are usually taken in duplicate with two matched calibrated probes as variability can occur. It is important that the patient rest in a climate controlled environment for 30 minutes before the TEWL measurement. If the patient is sweating, a steady state cannot be reached and the TEWL measurement cannot be accurately performed. If the skin must be washed before the TEWL measurement to remove cosmetics, it must be allowed to dry at least 30 minutes to prevent the externally applied water evaporation from affecting the measurement.
Q. How is skin hydration measured?
A. Another method of assessing skin barrier function is skin hydration. While transepidermal water loss (TEWL) measures the amount of water leaving the skin, skin hydration is an assessment of the water in the skin. My favorite method for measuring skin hydration is known as corneometry. I prefer to use a pin probe, which consists of a base to which several thick brass wires are attached. A small amount of electricity is delivered to the active wires, transmitted through the skin and registered by the receiving wires. Since water is the main conductor in the skin, the amount of electricity transmitted through the skin is an indirect indication of skin hydration. Higher corneometry readings indicate better skin moisturization.
Zoe Diana Draelos, M.D., is a clinical associate professor of dermatology, Wake Forest University School of Medicine, Winston-Salem, N.C., and primary investigator, Dermatology Consulting Services, High Point, N.C. Questions may be submitted via e-mail to firstname.lastname@example.org