for further investigation. Blood is often, but not always, found in the urine of menstruating females. This test is highly sensitive to hemoglobin and thus complements the

 

After dipping the strip, check the pH area. If the color on

the pad is not uniform, read the reagent area immediately,

comparing the darkest color to the appropriate Color Chart.

All reagent areas may be read between 1 and 2 minutes for

identifying negative specimens and for determination of

the pH and SG. Color changes that occur after 2 minutes

are of no diagnostic value. If using strips instrumentally,

the instrument will automatically read each reagent area

at a specified time.

Quality Control

For best results, performance of reagent strips should

be confirmed by testing known negative and positive

specimens or controls whenever a new bottle is first

opened. Negative and positive specimens or controls

may also be randomly hidden in each batch of specimens

tested. Water should not be used as a negative control.

Each laboratory should establish its own goals for

adequate standards of performance, and should question

handling and testing procedures if these standards are not

met. Chek-stix® Positive and Negative Control Strips, with

positive, negative or defined results, provide a convenient

basis for a urinalysis quality control program.

Because of the various constituents that are added to

commercial controls other than Chek-stix Control Strips,

or the way in which they are processed, specific gravity

values determined using Bayer Reagent Strips may not

always correspond with values given in the product inserts

for these controls.

Due to its specificity for acetoacetic acid, the ketone

reagent area may not react with commercial controls other

than Chek-stix Positive Control Strips. If questionable

results are obtained with the ketone reagent area, strip

reactivity should be checked with Chek-stix Positive

Control Strips or by testing negative and positive clinical

specimens that have been identified as positive or negative

with a reference test method.

Results

Results with Bayer Reagent Strips are obtained in

clinically meaningful units directly from the Color Chart

comparison when using strips visually. With instrumental

use, the reagent pads are “read” by the instrument and

the results are displayed or printed. The color blocks and

instrumental display values represent nominal values;

actual values will vary around the nominal values.

Limitations of Procedures

No laboratory tests, definitive diagnostic or therapeutic

decisions should be based on any single result or method.

Substances that cause abnormal urine color, such as drugs

containing azo dyes (e.g. Pyridium, Azo Gantrisin, Azo

Gantanol), nitrofurantoin (Macrodantin, Furadantin),

and riboflavin, may affect the readability of the reagent

areas on urinalysis reagent strips. The color development on

the reagent pad may be masked, or a color reaction may be

produced on the pad that could be interpreted visually and/

or instrumentally as a false positive.

Glucose

Ascorbic acid concentrations of 50 mg/dL or greater may

cause false negatives for specimens containing small

amounts of glucose (75–125 mg/dL). Ketone bodies

reduce the sensitivity of the test; moderately high ketone

levels (40 mg/dL) may cause false negatives for specimens containing small amounts of glucose (75–125 mg/

dL) but the combination of such ketone levels and low

glucose levels is metabolically improbable in screening.

The reactivity of the glucose test decreases as the SG of the

urine increases. Reactivity may also vary with temperature.

Bilirubin

Indican (Indoxyl sulfate) can produce a yellow-orange

to red color response that may interfere with the

interpretation of a negative or a positive bilirubin reading.

Metabolites of Lodine (etodolac) may cause false positive

or atypical results; ascorbic acid concentrations of 25 mg/

dL or greater may cause false negatives. Since very small

amounts of bilirubin may be found in the earliest phases

of liver disease, the user must consider whether the

sensitivity of Bayer Reagent Strips to bilirubin is sufficient

for the intended use.

Ketone

False positive results (Trace or less) may occur with highly

pigmented urine specimens or those containing large

amounts of levodopa metabolites. Compounds such as

Urine Analysis 75

mesna (2-mercaptoethane sulfonic acid) that contain

sulfhydryl groups may cause false positive results or an

atypical color reaction.

Specific Gravity

The chemical nature of the Bayer SG test may cause

slightly different results from those obtained with other

specific gravity methods when elevated amounts of

certain urine constituents are present. Highly buffered

alkaline urines may cause low readings relative to other

methods. Elevated specific gravity readings may be

obtained in the presence of moderate quantities (100–750

mg/dL) of protein.

Blood

Elevated specific gravity may reduce the reactivity of

the blood test. Clapoten (captopril) may also cause

decreased reactivity. Certain oxidizing contaminants,

such as hypochlorite, may produce false positive results.

Microbial peroxidase associated with urinary tract

infection may cause a false positive reaction. Levels of

ascorbic acid normally found in urine do not interfere

with this test.

pH

If proper procedure is not followed and excess urine

remains on the strip, a phenomenon known as “runover”

may occur, in which the acid buffer from the protein

reagent will run onto the pH area, causing a false lowering

of the pH result.

Protein

False positive results may be obtained with highly buffered

or alkaline urines. Contamination of the urine specimen

with quaternary ammonium compounds (e.g. from

some antiseptics and detergents) or with skin cleansers

containing chlorhexidine may also produce false positive

results.

Urobilinogen

The reagent area may react with interfering substances

known to react with Ehrlich’s reagent, such as

p-aminosalicylic acid and sulfonamides. Atypical color

reactions may be obtained in the presence of high

concentrations of ρ-aminobenzoic acid. False negative

results may be obtained if formalin is present. Strip

reactivity increases with temperature; the optimum

temperature is 22–26°C. The test is not a reliable method

for the detection of porphobilinogen. The absence of

urobilinogen cannot be determined with this test.

Expected Values

Expected values for the typical “normal” healthy

population and the abnormal population are listed below

for each reagent. Exact agreement between visual results

and instrumental results might not be found because of

the inherent differences between the perception of the

human eye and the optical systems of the instruments.

Glucose

The kidney normally excretes small amount of glucose.

These amounts are usually below the sensitivity of this test

but on occasion may produce a color between the negative

and the 100 mg/dL color blocks, and that is interpreted

by the instrument as a positive result. Results at the first

positive level may be significantly abnormal if found

consistently.

Bilirubin

Normally no bilirubin is detectable in urine by even

the most sensitive methods. Even trace amounts of

bilirubin are sufficiently abnormal to require further

investigation. Atypical colors (colors that are unlike the

negative or positive color blocks shown on the Color

Chart) may indicate that bilirubin derived bile pigments

are present in the urine sample and may be masking the

bilirubin reaction. These colors may indicate bile pigment

abnormalities and the urine specimen should be tested

further.

Ketone

Normal urine specimens ordinarily yield negative results

with this reagent. Detectable levels of ketone may occur

in urine during physiological stress conditions such

as fasting, pregnancy and frequent strenuous exercise.

In ketoacidosis, starvation or with other abnormalities of carbohydrate or lipid metabolism, ketones may

appear in urine in large amounts before serum ketone

concentrations are elevated.

Specific Gravity

Random urines may vary in specific gravity from

1.001–1.035. Twenty-four hour urines from normal adults

with normal diets and normal fluid intake will have a

specific gravity of 1.016–1.022.

Blood

The significance of the Trace reaction may vary among

patients, and clinical judgment is required for assessment

in an individual case. Development of green spots (intact

76 Concise Book of Medical Laboratory Technology: Methods and Interpretations erythrocytes) or green color (free hemoglobin/myoglobin)

on the reagent area within 60 seconds indicates the need

for further investigation. Blood is often, but not always,

found in the urine of menstruating females. This test is

highly sensitive to hemoglobin and thus complements the

microscopic examination.

pH

Both the normal and abnormal urinary pH range is from

5 to 9.

Protein

Normally, no protein is detectable in urine, although

the normal kidney excretes a minute amount. A color

matching any block greater than Trace indicates significant

proteinuria. For urine of high specific gravity, the test area

may most closely match the Trace color block even though

only normal concentrations of protein are present. Clinical

judgment is needed to evaluate the significance of Trace

results.