Creatinine clearance overestimated glomerular filtration rate in a heavy tea-drinker

To the Editor: It is known that methylxanthines, especially theophylline, increase production of urine and enhance excretion of water and electrolytes with a pattern similar to that produced by thiazides. The underlying mechanisms, which remain controversial, appear to be mainly mediated by antagonism of adenosine receptors 1 and by other effects on adenylate cyclase, phosphodiesterase and intracellular calcium. 2 A 29-year-old man was admitted to the hospital in January 2000 for an evaluation of his renal function. The patient had consumed large quantities of black Chinese and Nepalese tea and complained of heart throbbing and high blood pressure. On admission he had abnormally high creatinine and urea clearance and enhanced urinary excretion of creatinine, urea, sodium, and potassium (Table 1). He underwent abdomen ultrasonography, doppler imaging of renal arteries, and a kidney scan; no alterations were found. The glomerular filtration rate (GFR), as calculated by the method described by Gates, 3 appeared to be normal. Blood pressure was in the "high normal" range. An eye examination showed stage 1-hypertensive retinopathy. The patient's thyroid function, plasma renln activity, blood and urine aldosterone, and cathecolamines concentrations were in the normai range. The finding of decreased serum iron levels suggested impaired iron absorption due to the tannates contained in tea.4 We speculated that the increases of creatinine and urea clearance and of sodium and potassium urine excretion could be due to the effect of the large quantities of ingested tea; the estimated daily dose of methylxanthines during the last year was 250 to 300 nag caffeine and 5 to 6 mg theophylline (apart from tea, the patient's diet was poor in xanthine-containing foods). 5 Methylxanthines in normal subjects appear to inhibit solute reabsorption in both the proximal nephron and the diluting segments without changing either GFR or renal blood flow appreciably. 5 Accordingly, such a discrepancy between GFR and creatinine clearance in our patient could be reasonably ascribed to the increase of creatinine excretion. During his stay in the hospital, the patient was kept on a normal salt intake (6 to 8 g/day), xanthine-poor diet. This was followed by a decrease in urine volume, solute excretion, and free water clearance (Table 1). His palpitations ceased. The patient was discharged with the diagnosis of arterial hypertension (stage 1) and advised to decrease sodium intake and limit tea drinking.