Digoxin-phenytoin interaction.

Tolerance of Rifampicin SIR,-One of the virtues of rifampicin is said to be a good patient tolerance which, as you have noted (30 August, 1969, p. 487), should be particularly valuable in treatment of tuberculosis where tolerance is poor. To demonstrate objectively whether this good tolerance extends to domiciliary ambulant therapy we have used a simple urine test for rifampicin on patients attending the Dagenham Chest Clinic, consecutively during June, July, August, and September last year (Table). During the same period we examined urines of all patients on P. Butanol test for rifampicin.-This is derived from the biochemical method for estimation of rifampicin in urine, given in Lepetit Handbook.2 The depth of colour, which varies from salmon pink to dark orange, is a guide to urinary concentration of rifampicin. If rifampicin 10 mg./kg. of body weight has been ingested on an empty stomach two to eight hours previously, it is likely that the supernatant layer will be light orange or darker. The salmon pink colour may be found at 12 hours or even 24. A light orange corresponds to about 25 ug./ml. rifampicin; salmon pink to 2.5 tsg./ml. We base this statement on 240 urine samples taken from 11 patients over a period of three to five days, at two-hourly intervals from 7.30 a.m. to 7.30 p.m. False positives may occur in the presence of excess urinary bile pigments. Pines et al.3 suggested that rifampicin is obvious in untreated urine, but the colour is, in our experience, more orange than brown. However, simple inspection proved misleading to seven nurses in 82 out of 189 observations on eight "positive" and 19 control urines. Using the butanol test on these same urines reduced the error to 12 out of 189 observations. Although we agree that patient tolerance of rifampicin appears so far to be good, we suggest it may be wise to check from time to time that the drug is being taken. The above simple butanol test may help in this connexion.-We are, etc., SIR,-We would like to draw attention to what appears to be a toxic interaction of digoxin with phenytoin. Cardiac glycosides inhibit the Na,K-ATPase which is closely related to the metabolic "sodium pump". The inhibiting effects of ouabain and phenytoin on the Na,K-ATPase enzyme system are additive.' Phenytoin causes bra-dycardia2 as well as digoxin. A man with Down's syndrome, aged 53, began to get grand mal seizures once …