A Comparison of Urine Acidification of Two Loop Diuretics in Normal Volunteers: Torsemide vs Furosemide

Abstract

Because of the ready availability and low toxicity, furosemide has been widely used to test urine acidification despite somewhat low specificity (Reynolds TM et al., Br J Urol , 72:153-156, 1993 and Weger W et al., Nephrol Dial Transplant, 15:975-980, 2000). Realizing that torsemide, unlike furosemide, does not contain a sulfonamide moiety, which is responsible for the mild inhibitory effect on carbonic anhydrase by furosemide, we hypothesized that the former might be a better agent to test urine acification than the latter. In 8 normal volunteers (ages, 23 to 33), hourly urine samples were obtained for 4 hours following the administration of 40 mg of furosemide orally or intravenously. Tests were repeated in another day with 20 mg of torsemide given orally or intravenously. The baseline urine pH values were similar in the two groups (Mean SE: 6.2 0.2, oral and 6.2 0.2, iv, before the torsemide test and 6.3 0.3, oral and 6.3 0.2, iv, before the furosemide test). However, the lowest urine pH values were significantly lower following torsemide (4.9 0.1, oral and 4.9 0.1, iv) than following furosemide (5.6 0.2, oral and 5.5 0.2, iv) (p < 0.05). The urine pH remained greater than 5.5 in 4 of 8 patients after oral administration of furosemide, and 2 of 8 patients after iv adminitration. On the other hand, the urine pH were greater than 5.5 in none of 8 after oral administration of torsemide and 1 of 8 after iv administration of torsemide. If impaired urine acidification is defined as inability to reduce urine pH below 5.5, the specificity of the test is 50% and 75% respectively with oral and iv administration of furosemide, whereas the values are 100% and 88% after oral and iv administration of torsemide. In summary, torsemide appears to be a more reliable agent to test urine acidification, but sensitivity of the test needs to be determined in patients with impaired urine acidification.