Urinary acidifiers Methenamine

The nurse measures the fluid intake and output, especially when the primary health care provider orders an increase in fluid intake or when a kidney infection is being treated. The primary health care provider may also order daily urinary pH levels when methenamine or nitrofurantoin is administered. These drugs work best in acid urine failure of the urine to remain acidic may require administration of a urinary acidifier, such as ascorbic acid. 

Unlabeled uses Vitamin C (at least 2 g/day) may be used as a urinary acidifier in conjunction with methenamine therapy. 

Intermittent self-catheterization with or without a concomitant anticholinergic agent is recommended in patients with large postvoid urine residual volumes (>100 mL) or when the urinary problem is hyporeflexic in nature (failure to empty). Patients with large postvoid residual volumes are at risk for developing urinary tract infections and often are prescribed urinary acidifiers such as vitamin C or antiseptics such as methenamine mandelate to prevent infections. 

Methenamine and methenamine mandelate are only effective as urinary antisepties if the pH is about 5.5 or lower, when formaldehyde is released. This is normally achieved by giving urinary acidifiers such as ammonium chloride, ascorbic acid, or sodium acid phosphate. In the case of methenamine hippurate, the acidification of the urine is achieved by the presence of hippuric acid. The concurrent use of substances that raise the urinary pH such as acetazolamide, sodium bicarbonate, potassium or sodium citrate is clearly contraindicated. Potassium citrate mixture BPC has been shown to raise the pH by more than 1 at normal therapeutic doses, thereby making the urine sufficiently alkaline to interfere with the activation of methenamine to formaldehyde. Some antacids (containing magnesium, aluminium or calcium as well as sodium bicarbonate mentioned above) can also cause a significant rise in the pH of the urine. ... 

Methenamine mandelate, 1 g four times daily, or methen-amine hippurate, 1 g twice daily by mouth (children, 50 mg/kg/d or 30 mg/kg/d, respectively), is used only as a urinary antiseptic to suppress, not treat, urinary tract infection. Acidifying agents (eg, ascorbic acid, 4-12 g/d) may be given to lower urinary pH below 5.5. Sulfonamides should not be given at the same time because they may form an insoluble compound with the formaldehyde released by methenamine. Persons taking methenamine mandelate may exhibit falsely elevated tests for catecholamine metabolites. 

Methenamine is absorbed orally, but 10 to 30% decomposes in the gastric juice unless the drug is protected by an enteric coating. Because of the ammonia produced, methenamine is contraindicated in hepatic insufficiency. Excretion in the urine is nearly quantitative. When the urine pH is 6 and the daily urine volume is 1000 to 1500 mL, a daily dose of 2 g will yield a concennation of 18 to 60 pg/mL of formaldehyde this is more than the MIC for most urinary tract pathogens. Various poorly metabolized acids can be nsed to acidify the urine. Low pH alone is bacteriostatic, so acidification serves a double function. The acids commonly used are mandelic acid and hippuric acid (Uerx, Hiprex). 

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