Benedict's Test
Norm of Benedict's Test
Blue or negative.
Usage of Benedict's Test
Screening for disorders of glucose or amino acid metabolism.
Positive of Benedict's Test
Congenital deficiency of homogentisic acid, hyperglycemia, and melituria. Osmotic diarrhea in infants and children.
Color |
Glucose Quantitative |
Glucose Qualitative |
| Blue | 0 mmol/L | Negative |
| Green | 14 mmol/L | Trace |
| Greenish yellow precipitate | 28 mmol/L | 1+ |
| Yellow-green | 56 mmol/L | 2+ |
| Brown | 83 mmol/L | 3+ |
| Orange-red | >110 mmol/L | 4+ |
Description of Benedict's Test
A copper-reduction test that detects sugars, reducing substances, and homogentisic acid in the urine. Clients with a hereditary absence of homogentisic acid oxidase are unable to metabolize homogentisic acid, a by-product of phenylalanine and tyrosine metabolism. Some of the unmetabolized homogentisic acid is excreted in the urine, and the remainder causes ochronosis, a condition characterized by joint deposition of homogentisic acid that eventually causes joint degeneration. Benedict's solution is made of quantitative and qualitative reagent, each containing cupric sulfate dissolved in sodium sulfate but in different concentrations. When Benedict's solution is heated in the presence of sugar, homogentisic acid, or other reducing substances, the cupric sulfate is reduced and a yellow-to-red precipitate forms.
Professional Considerations of Benedict's Test
Consent form NOT required.
Preparation
- Obtain a clean container, two Pyrex test tubes, a wooden test-tube holder, a beaker, a Bunsen burner, a pipette, a dropper, and Benedict's solution.
Procedure
- Obtain a 10-mL random urine specimen in a clean container without preservative. A fresh specimen may be taken from a urinary drainage bag.
- Pipette 5 mL of Benedict's solution into a Pyrex test tube.
- Add 8 drops of urine.
- Mix the test tube gently.
- Using the wooden test-tube holder, boil the mixture over a Bunsen burner for 2 minutes and then cool to room temperature.
- Observe the color of the test-tube mixture and interpret the results as noted in the preceding table.
Postprocedure Care
- Refrigerate the specimen if the test is not performed immediately.
Client and Family Teaching
- The specimen must be free of stool and toilet tissue.
- Results are normally available within 24 hours.
Factors That Affect Results
- Results are most accurate on a freshly voided specimen.
- Glycosuria will cause a false-positive result when Benedict's solution is used for detection of homogentisic acid.
- Drugs and reducing substances that may cause false-positive results include ampicillin, ampicillin sodium, aminosalicylic acid, ascorbic acid, camphor, carbamazepine, cefaclor, cefadroxil monohydrate, cefamandole nafate, cefazolin sodium, cefonicid sodium, cefoperazone sodium, ceforanide, cefotaxime sodium, cefotetan disodium, cefoxitin sodium, ceftazidime, ceftizoxime sodium, ceftriaxone, cefuroxime axetil, cephalexin, cephaloridine, cephalothin sodium, cephradine, cephapirin sodium, chloral hydrate, chloramphenicol, chloroform, cimetidine, formaldehyde, fructose, galactose, glucosamine, glucuronic acid, homogentisic acid, levodopa, metaxalone, moxalactam disodium, nalidixic acid, nitrofurantoin, nitrofurantoin sodium, paraldehyde, penicillin G benzathine, penicillin G potassium, penicillin G procaine, phenol, probenecid, salicylates, streptomycin sulfate, sulfonamides, tetracyclines, turpentine, and uric acid.
Other Data
- Any reducing substance found in the urine may cause false-positive results.
- Use other methods of glucose measurement to help differentiate glucosuria from urine containing homogentisic acid.
