Does that cup of decaffeinated coffee give you a jolt? It may, because almost all decaf coffee contains some caffeine, a new University of Florida study shows.

The results could have implications for people told to avoid caffeine because of certain medical conditions such as high blood pressure, kidney disease or anxiety disorders, according to the study reported in this month's Journal of Analytical Toxicology.

"If someone drinks five to 10 cups of decaffeinated coffee a day, the dose of caffeine could easily reach the level in a cup or two of caffeinated coffee," said co-author Dr. Bruce Goldberger, a professor and director of the university's William R. Maples Center for Forensic Medicine.

Researchers purchased 10 cups of 453-gram drip-brewed coffee from nine national chains and local coffee houses and tested them for caffeine content.

Instant decaffeinated Folgers Coffee Crystals didn't have any caffeine, but the others contained caffeine ranging from 8.6 milligrams to 13.9 milligrams. Typically, 453 grams of drip-brewed coffee contain about 170 milligrams of caffeine.

Researchers also analyzed 12 samples of Starbucks decaffeinated espresso and brewed decaffeinated coffee. The espresso drinks had from three milligrams to 15.8 milligrams each, while the brewed coffee had from 12 to 13.4 milligrams per 453-gram drink.

Even moderate caffeine levels can increase heart rate, blood pressure, agitation and anxiety in some people, Goldberger said.

Dr. Roland Griffiths, a professor of behavioural biology and neuroscience at Johns Hopkins School of Medicine, said caffeine as low as 10 milligrams can cause behavioural effects in sensitive individuals. Some popular espresso drinks, such as lattes, can deliver as much caffeine as a can of Coca-Cola, about 31 milligrams.

"The important point is that decaffeinated coffee is not the same as caffeine-free," Griffiths said.