Week 3 - Technical Considerations 1

Why should urine specimens be well-mixed before testing?

Cellular elements settle; mixing prevents false negatives for blood and leukocyte esterase

What mixing method should be used before aliquoting urine?

Inversion

Why should urine be tightly sealed if not processed within 2 hours?

To prevent evaporation of ketones and bacterial overgrowth

What causes false-negative ketone results in urine testing?

Volatility of ketones; specimen left uncapped

What causes bacteria to produce nitrite and make urine alkaline?

Glucose consumption and ammonia production from bacterial metabolism

What temperature should urine be for accurate testing?

Room temperature

Why shouldn't cold urine be tested directly?

Cold temperature decreases enzyme activity (e.g., glucose oxidase)

What happens to glucose readings if a cold urine sample is used?

Possible false negative due to decreased glucose oxidase activity

Should reagent strips be refrigerated?

No

Why should reagent strips be protected from moisture and heat?

To maintain reactivity

What should you check before using a new lot of reagent strips?

Manufacturer's directions

What does discoloration on reagent strips indicate?

Loss of reactivity

How much time should pass before testing urine after collection?

As soon as possible

What kind of urine sample should be used for testing?

Well-mixed, unspun, room-temperature urine

What should you avoid when handling reagent strips?

Touching the test area with fingers

Why should volatile acids or fumes be avoided during testing?

They may affect strip readings

How long should the strip be dipped in urine?

No longer than 1 second

How should excess urine be removed from a strip?

Run edge along rim or blot on absorbent paper

Why should reagents not be allowed to run together on the strip?

Can cause inaccurate results

Should strips be placed on workbench surfaces directly?

No

What is the importance of exact timing during strip reading?

Each test has specific timing for accuracy

What should you correlate test results with?

Patient history and individual test findings

What is the principle used in the pH reagent strip?

Double Indicator System

What reagents are used for pH testing?

Methyl Red and Bromthymol Blue

Which parameter is least affected by external factors?

pH

What can cause pH runover errors?

Excess urine on pad + failure to blot

What happens when the protein buffer runs over into the pH pad?

Maintains a pH of 3.0; falsely acidic result

What is the critical error when holding urine strips vertically with excess urine?

Buffer system from protein pad contaminates pH pad

What is the principle behind the Protein test on reagent strips?

Protein Error of Indicators

What reagent is used for the Protein test?

Tetrabromphenol Blue and Buffer

What type of urine can cause a false positive for protein?

Highly alkaline urine

What happens when specific gravity is high in a protein test?

Can cause a false positive

Why can gross hematuria cause false positive in protein test?

Hemoglobin in blood reacts with the pad

How do QUATS (quaternary ammonium compounds) cause a false positive?

They alkalinize urine

What pigment can cause a false positive protein test?

Phenazopyridine (orange pigment)

Why does prolonged dipping cause a false negative in protein test?

Buffer may wash away

What types of proteins may lead to false negative in protein test?

Proteins other than albumin

What is microalbuminuria?

Albumin level below detection limit (20–200 mg/L)

What is the minimum level the dipstick protein pad can detect?

Approximately 300 mg/L

What is the principle behind the Glucose test on reagent strips?

Double Enzyme Sequential Assay

What reagents are used in the glucose test?

Glucose oxidase, peroxidase, KI or TMB

What kind of reaction is the glucose reagent test based on?

Oxidation-reduction reaction using a chromogen

What can cause a false positive glucose test?

Oxidizing agents like detergents

How does ascorbic acid cause a false negative glucose result?

Reduces chromogen, preventing color change

What is the effect of high vitamin C intake on glucose test?

May cause false negative due to ascorbic acid

What non-glucose substance can cause false negative in glucose test?

High levels of ketones

How does high specific gravity affect glucose test?

Inhibits enzyme reaction

How does low temperature affect glucose test?

Decreases enzyme activity

Why do unpreserved urine samples give false negative glucose results?

Bacteria consume glucose over time

What is the principle behind the Ketone test on reagent strips?

Nitroprusside Reaction

What reagents are used for ketone testing?

Sodium nitroprusside and Glycine

What can cause a false positive in the ketone test?

Highly pigmented red urine, phthalein dyes, levodopa, or sulfhydryl-containing medications

Why do sulfhydryl medications cause a false positive in ketone test?

They react with sodium nitroprusside

What is the main cause of false negative in ketone testing?

Improperly preserved urine due to volatility of ketones

What is the principle behind the Blood test on reagent strips?

Pseudoperoxidase Activity of Heme

What reagents are used for the blood test?

Hydrogen peroxide and Tetramethylbenzidine

What causes a false positive result in the blood reagent test?

Oxidizing agents, bacterial peroxidase, or menstrual contamination

How can E. coli cause a false positive blood result?

It produces bacterial peroxidase

Why does menstrual contamination cause false positive blood result?

Menstrual blood mixes with urine

How does ascorbic acid affect the blood reagent test?

Inhibits chromogen oxidation → false negative

Why does high specific gravity cause false negative blood results?

It crenates RBCs (shriveling them)

What is the effect of high nitrite on blood test result?

Interferes with oxidation reaction → false negative

Why should the urine specimen be mixed before testing for blood?

Unmixed urine may miss settled RBCs → false negative

What preservative can inhibit the blood test reaction?

Formalin

What drug can cause false negative results in blood reagent testing?

Captopril