[Studies of free theophylline in asthmatic patients]

Total and free theophylline concentrations were examined by fluorescent immunoassay using serum from 48 asthmatic patients treated with sustained-release theophylline formulation. Total and free theophylline concentrations showed a good correlation. The mean % free theophylline (free theophylline concentration X 100/total theophylline concentration) was 44.7% in children and 43.6% in adults. The % free theophylline had no relation to age of the patients or total theophylline concentration. However, the % free theophylline increased correlatively when serum pH or albumin was decreased. As changes in free theophylline concentration influence therapeutic responses and side effects, an assessment of serum pH and albumin in patients treated with theophylline is necessary.