SI units and anticonvulsant drugs in paediatrics.

The concept of using SI units has now been accepted in principle by most medical practitioners in the United Kingdom, and laboratories now report most of their results in these units. During the transition period the use of conversion nomograms such as those produced by Baron' and Bold and Wilding2 have proved invaluable in helping both clinician and laboratory worker to adjust to new reference ranges. In drug monitoring the change has not been universal, and the reasons are so complex that uniformity in the near future seems unlikely. The lack of uniformity is not entirely due to the introduction of SI units, as several gravimetric units are in use-for example, ,Lg/ml, mg/100 ml, mg/l. A medical student may have learnt to measure drugs in one of these units and then find that in the hospital where he does his resident posts SI or one of the other gravimetric systems is used. We have constructed a conversion nomogram for gravimetric and molecular units to avoid the dangers of misinterpreting drug concentrations. The nomogram has been used in the paediatric department at Northwick Park Hospital for over a year. more common drugs. This service is used by the paediatric department at Northwick Park Hospital. St Albans City Hospital reports the results in gravimetric units (,ug/ml), as these are the most acceptable units to most of its users, whereas North-wick Park Hospital reports the results of all tests performed locally in molecular units (,umol/l). These results may be converted easily by the use of the nomogram (see figure). A paediatric therapeutic range has been included on the nomogram so that the clinician can decide at a glance whether his treatment is appropriate at the same time as he converts the results. These ranges are based on considerable experience in dealing with specimens at St Albans City Hospital and have been substantiated by experience at Northwick Park Hospital. The recommended range for valproate is based on concentrations in specimens collected two to four hours after dosage.