September 2017

Causes for Urinalysis Discrepancies

Routine urinalysis remains a fundamental tool to alert healthcare providers to the presence of renal damage and/or urinary tract disease. However, discrepancies between routine urinalysis and microscopy or patient history may occur due to physiological factors, medications, or collection issues. In an effort to assist with the interpretation of routine urinalysis results, the table below lists some circumstances when false-positive and false-negative results may occur. In all cases, results must be correlated with patient history and other clinical information.

Causes of False-Positive and False-Negative Routine Urinalysis Results

Dipstick test False positive False negative

High specific gravity, ascorbic acid, high ketones, improperly Alkaline urine (pH>9), strong stored specimens (i.e. glycolysis) Glucose oxidizing agents (e.g. bleach), or in the presence of peroxide contaminants acetylcysteine, captopril, mesna, curcuma

Highly coloured substances that Ascorbic acid, acetylcysteine, mask results such as boric acid, hypochlorite, phenazopyridine (Pyridium), captopril, mesna, curcuma, citric Bilirubin indican-indoxyl sulfate acid, chlorhexidine, oxalic acid, (indicans), large quantities of high nitrite concentrations, chlorpromazine metabolites or improper storage or light Lodine metabolites exposure (oxidizes bilirubin)

Free-sulfhydryl drugs (e.g. Mesna, captopril, N- Delay in examination of urine or acetylcysteine), highly Ketones improper storage (bacterial pigmented urines, atypical breakdown), in the presence of Detects acetoacetate, does colours with phenylketones and boric acid, formalin, not react with acetone or phthaleins, large quantities of hypochlorite, meropenem, or beta hydroxybutyrate. levodopa metabolites, curcuma, Lodine imipenem, or hydrochlorothiazide

Dextran solutions, IV Specific Gravity Falsely low if alkaline urine radiopaque dyes, proteinuria,

Dipstick test False positive False negative ketoacidosis (pH≥6.5)

Blood Captopril (Capoten) and other Detects hemoglobin compounds containing sulfhydryl peroxidase-like activity groups, high specific gravity, from either intact or lysed ascorbic acid (high vitamin C erythrocytes (RBCs). Lysed Peroxidases (e.g. microbial), concentration), formalin, high erythrocytes may produce strong oxidizing agents nitrite, acetylcysteine, quinidine, apparent discrepancies cefoxitin, levodopa, mesna, between positive dipstick Keflin, curcuma, Lodine, and negative microscopic results. Equally specific for hydrochlorothiazide, metformin, hemoglobin and myoglobin. chlorhexidine, or chloroquine

No interferences known; unaffected by protein concentration. pH Bacterial growth may cause a marked alkaline shift (pH>8.0) because of urea conversion to ammonia.

Highly buffered or alkaline urine (pH>9), such as alkaline drugs, improperly preserved specimen, contamination with quaternary ammonium compounds, highly Primary protein is not albumin coloured substances that mask Protein (e.g. Bence-Jones protein), in results, such as drugs the presence of curcuma (phenazopyridine) or beet ingestion, antiseptic agents, chlorhexidine or in the presence of quinidine, chloroquine, or Lodine

Elevated nitrite levels, phenazopyridine, any other Formalin, improper storage, Ehrlich’s reactive substance resulting in oxidation to urobilin, (e.g. porphobilinogen, indicans), acetylcysteine, captropril, atypical colours caused by hypochlorite, mesna, Tagamet, Urobilinogen sulfonamides, p-aminobenzoic curcuma, Lodine, acid, p-aminosalicylic acid, sulfamethoxazole, chlorhexidine, highly coloured substances that glucose, hydrochlorothiazide, mask results, such as drugs lactose, meropenem, or (phenazopyridine) or beet nitrofurantoin ingestion, methyldopa,

Dipstick test False positive False negative procaine, chlorpromazine

High specific gravity, ascorbic acid, oxalic acid, Lodine, formalin, chlorhexidine, various factors that inhibit or prevent Highly coloured substances that nitrite formation despite mask results, such as drugs bacteriuria (e.g. nitrate Nitrites (phenazopyridine), curcuma or reductase-negative bacteria, lack beet ingestion, improper storage of urine nitrate, presence of with bacterial proliferation antibiotics, insufficient time for bacteria to reduce nitrate, or large quantities that convert nitrite to nitrogen)

Leukocyte esterase High specific gravity, glycosuria, Detects esterase activity ketonuria, proteinuria, oxalic from either intact or lysed Highly coloured substances that acid, ascorbic acid, boric acid, granulocytic leukocytes; mask results (e.g. drugs strong oxidizing agents, lymphocytes do not produce a positive reaction. [phenazopyridine], beet quinidine, Tagamet, glycine, Lysed granulocytic ingestion), vaginal chloroquine, sulfamethoxazole, leukocytes may produce contamination of urine, formalin chlorhexidine, nitrofurantoin, apparent discrepancies or curcuma Lodine, drugs such as between positive dipstick tetracycline, gentamicin, and and negative microscopic cephalosporins results.

For further information regarding urinalysis testing, please contact the undersigned; Hui Li, Ph.D., FCACB, DABCC, FACB Dana Bailey, Ph.D., FCACB [email protected] or 1.800.668.2714 x5208 [email protected] or 1.800.668.2714 x1220

References 1. Jeff A. Simerville, M.D., William C. Maxted, M.D., and John J. Pahira, M.D. Urinalysis: A Comprehensive Review. Am Fam Physician. 2005 Mar 15;71(6):1153-1162. 2. Roberts JR. Urine Dipstick Testing: Everything You Need to Know. Emergency Medicine News. 2007 Jun 1;29(6):24-7. 3. Susianti H, Lie S, Yoavita. Auto-Identification of Discrepancies between Urine Test Strip and Sediment Results Using Cross Check Function on Fully Automated Urine Analyzer. Int Clin Pathol J. 2015 Dec 1(4): 00020. 4. Brunzel, N.A. Fundamentals of Urine and Body Fluid Analysis. 2013 5. King Strasinger S and Schaub Di Lorenzo M. Urinalysis and Body Fluids 6th Ed. 2014