Middlesex claim the spinner ``simply forgot'' to give a sample during the championship match with Essex at Chelmsford.
Team-mate Paul Weekes gave his at lunchtime but Tufnell left the ground without reporting to the testers.
``Phil simply forgot to take the test and drove off from the ground,'' Middlesex secretary Joe Hardstaff has been reported as saying.
``Middlesex consider this an extremely serious business. I don't know all the facts because Phil is away but I understand he has a perfectly logical ex planation.''
Tufnell, chosen for England's tour to the Caribbean this winter, could face a disciplinary panel if his explanation to the England Cricket Board officials is viewed as unsatisfactory.