|Description||Theophylline relaxes bronchial smooth muscles and is used to prevent or treat asthma. It is rapidly cleared from the circulation (half life, non-smoker = 3-4 hrs, half life, smoker = 9 hr). In neonates the half life is prolonged (20-30 hr) due to immaturity of the liver. Patients with chronic heart failure also tend to have increased half lives. The concentration of theophylline is proportional to the forced expiratory volume; clearance is a function of a metabolic process that is dose dependent. Hence a small dose increase can cause a disproportionately large increase in serum concentration and intoxication. Side effects include sweating, tremor, vomiting, pupil dilation and hyperventilation.|
|Indication||Therapeutic Drug Monitoring.
|Additional Info||For monitoring purposes, blood should not be drawn until a steady state has been attained. This will not be achieved until 24-48 hours after starting therapy or changing dose. In neonates the period may be as long as 5-6 days. Blood should be taken for trough values before a dose. During infusions, blood should be taken after stopping the infusion for 15 min. There is a circadian rhythm of theophylline metabolism, resulting in higher morning trough levels than later in the day. Repeat blood sampling must be done at the same time each day. Theophylline metabolism is associated with large intraindividual variations and levels should be closely monitored.|
|Interpretation||FACTORS WHICH INCREASE LEVELS: a) type of formulation -peak concentrations range from 115 mins with elixirs to 2hr for ordinary formulations and 4 hrs for the modified release preparations b) Age - neonates and the elderly c) Diet - high carbohydrate, low protein, methylxanthines d) Drugs including macrlide antibiotics, allopurinol, cimetidine, oral contraceptives, propranalol.
FACTORS WHICH DECREASE LEVELS: a) Age - children b) Diet - low carbohydrate, high protein c)drugs - carbamazeopine, phenobarbitone, phenytoin, rifampacin, sulphinpyrazone.|
|Tube||Serum or Heparin|
|Collection Conditions||Use Gold top (serum gel) tube except for Intensive care units, renal unit, transplant unit, patients on IV heparin (use Green/Yellow top tube for these patients).|
|Min. Vol||1 mL|
|Ref. Range (Male)||10 - 20 mg/L (>4 wks)|
|Ref. Range (Female)||10 - 20 mg/L (>4wks)|
|Ref. Range (Paed)||Neonates (0-4wks) 5 - 10 mg/L|
|Ref. Range Notes||NOTE: Reporting units changed from mmol/L to mg/L May 1, 2007
|IP Acute TAT||Refer to Website|
|IP Routine TAT||Refer to Website|
|GP Acute TAT||- Contact Laboratory|
|GP Routine TAT||Refer to Website|
Originally edited by : MC. Review due on 21/06/2017 12:06:10. Published By Sylvia Bennett on 21/06/2016 12:06:10.