Correspondence

False Positive Tests for Urinary Ketones

N Engl J Med 1994; 330:578February 24, 1994

Article

To the Editor:

Patients receiving total parenteral nutrition may undergo routine bedside analysis of urine for glucose and ketones with the use of reagent strips (Ketodiastix, Labstix, or Multistix; Miles, Elkhart, Ind.). Two patients who had undergone urinary diversion procedures (creation of a Miami pouch and cecostomy) and were receiving total parenteral nutrition had positive tests for urinary ketones. Their serum glucose concentrations were less than 150 mg per deciliter; other causes of ketonuria were ruled out. Neither patient was receiving a medication known to interfere with this test1 (and Customer Service Department, Miles: personal communication). The serum of one patient tested negative for ketones according to the Acetest method (Miles).

The urinary-diversion pouches of both patients were being irrigated with 6 percent acetylcysteine (70 ml every six hours), used as a mucolytic agent. Ileal and colonic segments used in reconstructing the lower urinary tract retain their function of secreting mucus, which may result in obstruction of the pouch by inspissated mucus2. Acetylcysteine has mucolytic properties because it can reduce disulfide linkages of mucoproteins through an interchange reaction. Long-term irrigation of urinary-diversion pouches with this agent has been recommended3.

The irrigating solution was responsible for the positive tests for urinary ketones. It was positive for ketones when tested with reagent strips. When added to 100 ml of ketone-negative urine, 10 ml of the solution produced a trace positive reaction (5 mg per deciliter); 20 ml produced a substantial positive reaction (80 mg per deciliter).

The urinary ketone test uses sodium nitroferricyanide (sodium nitroprusside). Acetoacetic acid induces a color change (to lavender) by reducing the iron in the nitroferricyanide to its ferrous state. Acetylcysteine can also reduce the iron moiety, causing a false positive test for urinary ketones. Irrigating solutions containing acetylcysteine should be added to the list of medications and substances that can result in such false positive tests.

Beverly J. Holcombe, Pharm.D.
Angela M. Hopkins, M.D.
William D. Heizer, M.D.
University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7080

3 References

1. 1

   Knoben JE, Anderson PO. Handbook of clinical drug data. 6th ed. Hamilton, Ill.: Drug Intelligence Publications, 1987.
   

2. 2

   Murray K, Nurse DD, Mundy AR. Secreto-motor function of intestinal segments used in lower urinary tract reconstruction. Br J Urol 1987;60:532-535
   CrossRef | Medline

3. 3

   Sheffner AL. The reduction in vitro in viscosity of mucoprotein solutions by a new mucolytic agent, N-acetyl-l-cysteine. Ann N Y Acad Sci 1963;106:298-310
   CrossRef | Web of Science | Medline