Reduced renal phosphate threshold concentration in alcoholic cirrhosis

Hypophosphatemia is frequently found in patients with alcoholic cirrhosis, and its postulated causes include inadequate phosphorus (PO4) intake, reduced gastrointestinal absorption, and inappropriate PO4 loss in the urine. Of these, the first two would be expected to be associated with a high threshold concentration (Tm/GFR) and the last with a low Tm/GFR. 24 patients with alcoholic cirrhosis were studied. All had normal renal function. Simultaneous mid-morning serum and urine samples were obtained and analyzed for creatinine, Ca, PO4, and Mg. Median serum PO4 was 2.75 mg/dl, including 8 patients who were hypophosphatemic (serum PO4 less than 2.5 mg/dl). The median Tm/GFR was 2.3 mg/dl; 13 of the patients had values below 2.5 mg/dl. There was a significant correlation between serum PO4 and Tm/GFR (p less than 0.001). The median serum Ca was 8.5 mg/dl and did not correlate with Tm/GFR. The median serum Mg was 2.0 mg/dl. Serum magnesium was significantly correlated with Tm/GFR (p less than 0.05). We conclude that hypophosphatemia in alcoholic cirrhosis is associated with a low Tm/GFR and that this may be related to hypomagnesemia.