Problem 2 - Clinical Chemistry

Question

Which of the following choices is false concerning blood obtained by heel stick (capillary) and venipuncture (venous)? a. The chemical composition of the sera derived from each is identical. b. The capillary specimen is likely contaminated with interstitial fluid and tissue debris. c. Venous blood contains higher bilirubin and calcium concentrations. d. Capillary blood contains less concentrated proteins due to mixing with interstitial fluid.

Step-by-Step Solution

Answer

a

Step 1 - Analyze Choice (a)

Consider the statement: 'The chemical composition of the sera derived from each is identical.' This statement is false because capillary blood and venous blood can have different chemical compositions due to the presence of interstitial and intracellular fluid in capillary blood, which may alter the levels of certain analytes.

Step 2 - Analyze Choice (b)

Consider the statement: 'The capillary specimen is likely contaminated with interstitial fluid and tissue debris.' This statement is true. Capillary blood is often mixed with interstitial fluid, especially in children, due to the nature of the collection process.

Step 3 - Analyze Choice (c)

Consider the statement: 'Venous blood contains higher bilirubin and calcium concentrations.' This statement is generally true, as the composition of venous blood can have higher concentrations of certain substances compared to capillary blood.

Step 4 - Analyze Choice (d)

Consider the statement: 'Capillary blood contains less concentrated proteins due to mixing with interstitial fluid.' This statement is true as interstitial fluid dilutes the concentration of proteins in capillary blood.

Step 5 - Determine the False Statement

After analyzing all the choices, choice (a) is determined to be false because the chemical composition of sera derived from capillary and venous blood is not identical.

Key Concepts

Blood Specimen Collection

Collecting blood samples is a common medical procedure that allows healthcare professionals to understand a patient’s health. There are two primary methods for collecting blood: from a vein (venous blood) and from a finger or heel stick (capillary blood).

Each method has its use depending on the situation. For instance, venous blood is often used for more detailed lab tests, while capillary blood is typically used for quick assessments or when only a small sample is needed.

The process involves adhering to proper protocols to avoid contamination, ensuring accurate results. Healthcare workers must wear gloves, use sterile equipment, and follow specific guidelines for each collection method.

Capillary Blood

Capillary blood is obtained by pricking the skin, often the finger or heel. This method allows healthcare workers to collect a small amount of blood.

Advantages:

• Less invasive and often less painful than venipuncture.
• Quick and convenient for small samples.

However, due to the nature of the collection, capillary blood can be mixed with interstitial fluid, which is the fluid between the cells. This can affect the accuracy for some measurements, making it crucial to consider what tests are appropriate for this type of sample.

Venous Blood

Venous blood is collected from a vein, usually in the arm. This method provides a larger and often purer blood sample because it is less likely to be mixed with other bodily fluids.

Advantages:

• More accurate for certain tests, such as measuring bilirubin and calcium concentrations.
• Can be used for a wide range of diagnostic tests due to its volume and purity.

Venous blood is usually preferred when extensive laboratory work is needed, providing more reliable data compared to capillary samples.

Chemical Composition

The chemical composition of capillary and venous blood can differ. Capillary blood, being a mix of blood from capillaries, arterioles, and venules, includes interstitial and sometimes intracellular fluids that can affect concentrations of certain substances.

Capillary blood often shows:

• Lower protein concentrations due to the dilution effect of interstitial fluid.
• Potential contamination with tissue debris.

Venous blood, being drawn directly from veins, tends to have a more consistent composition ideal for many laboratory tests. Higher bilirubin and calcium levels in venous blood are typical, providing clearer diagnostic markers for various conditions.