Unit 3: Quantitative Tests - Subjective Questions

Principle: The Dubois method, also called the phenol–sulfuric acid method, is based on the conversion of carbohydrates into furfural or hydroxymethylfurfural derivatives by concentrated sulfuric acid. These products react with phenol to form a yellow-orange colored complex.

• The intensity of the color is directly proportional to the amount of carbohydrate present.
• The absorbance is usually measured at approximately for hexoses.
• A standard carbohydrate solution, commonly glucose, is used to prepare a calibration curve.
• The concentration of carbohydrate in the unknown sample is obtained by comparing its absorbance with the standard curve.

Procedure:

1. Prepare a series of known glucose standards covering the expected concentration range.
2. Pipette the standards and unknown sample into appropriately labeled test tubes.
3. Add the required volume of phenol reagent to each tube.
4. Rapidly add concentrated sulfuric acid down the side of each tube.
5. Mix carefully and allow the tubes to stand for color development.
6. Incubate or cool the tubes according to the laboratory protocol.
7. Measure absorbance at approximately against a reagent blank.
8. Plot absorbance against carbohydrate concentration for the standards.
9. Determine the concentration of the unknown from the calibration curve, applying any dilution factor.

Reagent blank:

• Contains all reagents except the carbohydrate-containing sample.
• Corrects for the absorbance contributed by phenol, sulfuric acid, impurities, and the cuvette.
• The blank absorbance is subtracted from sample and standard readings when required.

Standard curve:

• It is prepared using known concentrations of a carbohydrate standard.
• Absorbance is plotted on the -axis and concentration on the -axis.
• In the working range, the relationship is approximately linear:

where is absorbance, is carbohydrate concentration, is the slope, and is the intercept.

• The concentration of the unknown is calculated from its absorbance and corrected for dilution.

Important precautions:

• Concentrated sulfuric acid must be handled carefully because it is highly corrosive and generates heat on mixing.
• Add acid consistently and use the same order of reagent addition for all tubes.
• Use clean, dry glassware and calibrated pipettes.
• Prepare standards accurately and avoid contamination between tubes.
• Mix all tubes uniformly and allow equal time for color development.
• Measure absorbance within the specified time and wavelength.
• Dilute samples whose absorbance lies outside the standard curve range.

Sources of error:

• Incorrect pipetting or dilution.
• Unequal reaction times or temperatures.
• Turbidity, interfering substances, or colored sample components.
• Degraded phenol or improperly prepared standards.
• Scratched or dirty cuvettes and instrument drift.

Given:

For the sample, .

Therefore, the concentration in the diluted sample is .

The original sample was diluted tenfold:

Answer: The original carbohydrate concentration is , or .

Principle: The Lowry method involves two reactions.

• In an alkaline medium, peptide bonds of proteins react with copper ions to form a violet-colored copper–protein complex. This is the biuret reaction.
• The copper-treated protein then reduces the phosphomolybdic–phosphotungstic components of the Folin–Ciocalteu reagent.
• Amino acids such as tyrosine and tryptophan contribute substantially to the reduction reaction.
• A blue-colored product is formed, and its absorbance is measured usually near .
• The color intensity is proportional to the protein concentration within the working range.

A standard protein, commonly bovine serum albumin, is used to construct a calibration curve.

Procedure:

1. Prepare a series of protein standards, usually using bovine serum albumin.
2. Pipette standards, unknown samples, and a reagent blank into labeled tubes.
3. Add the alkaline copper reagent to each tube.
4. Mix thoroughly and allow the tubes to stand for the specified time.
5. Add diluted Folin–Ciocalteu reagent rapidly and mix immediately.
6. Incubate the tubes for color development under standardized conditions.
7. Measure absorbance at approximately against the blank.
8. Plot absorbance against standard protein concentration.
9. Read the unknown concentration from the standard curve and multiply by the dilution factor, if applicable.

Alkaline copper reagent:

• Provides an alkaline environment for the reaction.
• Copper ions react with peptide bonds to produce a copper–protein complex.
• This initial reaction is related to the number of peptide bonds in the protein.

Folin–Ciocalteu reagent:

• Contains phosphomolybdate and phosphotungstate complexes.
• It is reduced by the copper–protein complex and mainly by aromatic amino acid residues such as tyrosine and tryptophan.
• Reduction produces a blue-colored compound.
• The absorbance of this compound is measured spectrophotometrically and is proportional to protein concentration under controlled conditions.

Thus, Lowry's method is more sensitive, but it requires stricter control of interfering substances and reaction conditions.

Limitations and interferences:

• Reducing agents such as dithiothreitol, mercaptoethanol, and ascorbate may reduce the Folin reagent and produce falsely high results.
• Detergents, chelating agents, strong acids, and some buffers can interfere with color development.
• The response differs among proteins because the method depends partly on tyrosine and tryptophan content.
• The color is not completely stable indefinitely, so absorbance should be read at a consistent time.
• The method is nonlinear at high concentrations and requires dilution when necessary.

Precautions:

• Use freshly prepared or properly stored reagents.
• Add the Folin reagent rapidly and mix immediately.
• Maintain identical incubation times and temperatures.
• Use a suitable blank containing all reagents except protein.
• Prepare standards in a matrix similar to that of the unknown sample whenever possible.

Given:

For the unknown, .

Thus, the concentration in the assayed diluted solution is .

Applying the fivefold dilution factor:

Answer: The original protein concentration is , equivalent to .

Principle: Glucose oxidase specifically oxidizes --glucose in the presence of oxygen and water to form gluconic acid and hydrogen peroxide:

The hydrogen peroxide is then used in a second reaction. In the presence of peroxidase, it oxidizes a chromogenic compound to produce a colored product:

The intensity of the color is proportional to the glucose concentration and is measured spectrophotometrically, commonly near , depending on the chromogen system.

Procedure:

1. Collect blood using an appropriate anticoagulant when plasma or whole blood is required; serum may also be used according to the protocol.
2. Separate plasma or serum promptly and avoid hemolysis.
3. Prepare glucose standards and a reagent blank.
4. Add the specified volume of sample, standard, and blank to labeled tubes.
5. Add glucose oxidase–peroxidase reagent to each tube.
6. Mix and incubate for the specified time and temperature.
7. Measure the absorbance of the colored product at the recommended wavelength, commonly around .
8. Calculate glucose concentration by comparison with the standard:

9. Apply any dilution factor and report the result in or .

After blood collection, living cells continue to metabolize glucose through glycolysis. If the sample is not processed promptly, the measured glucose concentration may decrease and produce a falsely low result.

Important considerations:

• Separate serum or plasma from cells as soon as possible.
• Use an appropriate antiglycolytic preservative, such as fluoride, when immediate separation is not possible.
• Avoid hemolysis because cell components can interfere with the assay.
• Store samples at the recommended temperature and analyze them within the stated stability period.
• Record whether the sample is fasting, postprandial, or random because physiological status affects blood glucose.
• Use clean, labeled containers and avoid contamination with glucose-containing fluids.

Fasting blood glucose:

• Measured after an overnight fast, usually for approximately – hours.
• Helps assess basal glucose regulation and is commonly used in screening for diabetes mellitus.

Postprandial blood glucose:

• Measured at a defined time, commonly two hours after a meal.
• Provides information about the body's ability to control the rise in glucose after food intake.

Random blood glucose:

• Measured at any time without regard to the last meal.
• Useful when symptoms of hyperglycemia are present or for rapid clinical assessment.

Interpretation must consider the patient's clinical condition, medications, sampling time, and the reference range of the laboratory. A single result should not be interpreted without appropriate clinical correlation.

Potential sources of interference include:

• Reducing substances: Ascorbic acid, uric acid, bilirubin, and some drugs may react with hydrogen peroxide or the chromogen and cause inaccurate results.
• Hemolysis: Hemoglobin can interfere with spectrophotometric measurement.
• Delayed processing: Cellular glycolysis lowers glucose concentration.
• Improper temperature or incubation time: These alter enzyme activity and color development.
• Lipemia or turbidity: Scattering of light can increase apparent absorbance.
• Incorrect wavelength: The assay must be read at the wavelength specified for the chromogen.
• Poor reagent storage: Loss of glucose oxidase or peroxidase activity produces unreliable results.

Using a suitable blank, controls, proper sample handling, and validated reagents minimizes these errors.

The comparative formula is:

Substituting the values:

Answer: The glucose concentration in the blood sample is .

To convert to SI units:

All three methods require accurate pipetting, suitable blanks and standards, controlled incubation, and measurement within the validated linear range.

Calibration curve: A calibration curve relates the measured absorbance to known analyte concentrations. It allows the concentration of an unknown to be obtained by interpolation.

Linearity: Within a specific range, absorbance should be proportional to concentration:

At concentrations above the linear range, the response may become nonlinear because of reagent depletion, instrument limitations, or chemical effects. Results should not be calculated from an invalid range.

Dilution: If an unknown sample gives an absorbance above the standard range, it should be diluted with an appropriate solvent or buffer and retested. The original concentration is then calculated as:

These practices improve accuracy, precision, and reliability.

Quality-control measures include:

• Run a reagent blank to correct for background absorbance.
• Analyze at least one known control sample with each batch.
• Prepare standards accurately and verify that the calibration curve is linear over the working range.
• Perform samples in duplicate when possible to assess precision.
• Use calibrated pipettes, clean cuvettes, and a properly functioning spectrophotometer.
• Maintain consistent reagent volumes, incubation times, temperatures, and wavelengths.
• Reject results when control values fall outside the accepted range.
• Record reagent lot numbers, preparation dates, sample identity, absorbance values, calculations, and dilution factors.
• Investigate unexpected results by checking sample integrity, reagent condition, instrument performance, and calculation steps.