Problem 12 - Clinical Chemistry

Question

Measuring serum ammonia levels has the potential to be fraught with preanalytical errors that may interfere with achieving an accurate result. Of the following preanalytical steps, which is incorrect? a. After phlebotomy, the patient's blood should be immediately placed on ice. b. The blood should be collected in a red clot tube without anticoagulant. c. Hemolyzed samples should be rejected as this interferes by falsely increasing ammonia levels. d. Lipemia may also interfere with plasma ammonia measurements. e. All of the above are correct.

Step-by-Step Solution

Answer

Option B is incorrect.

Step 1 - Understanding the Question

Read the question carefully. It asks which preanalytical step is incorrect when measuring serum ammonia levels. All provided options must be evaluated to find the incorrect step.

Step 2 - Reviewing Option A

Option A: Immediately placing the patient's blood on ice after phlebotomy is correct. This step is necessary to prevent the degradation of ammonia which could lead to inaccurate measurements.

Step 3 - Reviewing Option B

Option B: Blood collection for ammonia measurement should be done in a tube containing an anticoagulant, such as EDTA, and not in a red clot tube without anticoagulant. Therefore, this step is incorrect.

Step 4 - Reviewing Option C

Option C: Hemolyzed samples should be rejected because hemolysis can falsely increase ammonia levels. This statement is correct.

Step 5 - Reviewing Option D

Option D: Lipemia can interfere with plasma ammonia measurements. This statement is correct.

Step 6 - Reviewing Option E

Option E: This option states that all of the above are correct, which contradicts the finding that option B is incorrect. Therefore, option E is incorrect.

Step 7 - Concluding the Answer

Based on the analysis, the incorrect preanalytical step is option B: The blood should not be collected in a red clot tube without anticoagulant.

Key Concepts

preanalytical errors

Preanalytical errors refer to mistakes that occur before analyzing a clinical sample. In serum ammonia measurement, these errors can significantly impact the accuracy of the results. Common preanalytical errors include improper sample collection, incorrect handling and storage, and using incorrect containers.

To minimize preanalytical errors, it's crucial to:

• Follow strict phlebotomy procedures.
• Use the appropriate anticoagulant tubes.
• Avoid contamination by hemolysis or lipemia.
• Handle and store samples correctly, such as placing them on ice immediately after collection.

phlebotomy procedure

Phlebotomy, the process of drawing blood, must be done carefully to ensure accurate serum ammonia measurements. The steps include:

• Using clean and sterile equipment to avoid contamination.
• Collection in the appropriate tube with an anticoagulant like EDTA, not a red clot tube.
• Quickly placing the sample on ice to prevent degradation of ammonia.
• Drawing blood gently to prevent hemolysis.

Following these steps helps in getting reliable results.

hemolysis impact

Hemolysis occurs when red blood cells break down, releasing their contents into the serum. This can artificially elevate ammonia levels, impacting serum ammonia measurements.

To avoid hemolysis:

• Handle the blood sample gently.
• Avoid vigorously shaking the tube.
• Ensure proper needle size and technique during blood draw.
• Reject hemolyzed samples to prevent inaccurate readings.

lipemia interference

Lipemia is the presence of a high concentration of lipids in the blood, which can interfere with various laboratory tests, including serum ammonia measurements.

High lipid levels in the sample can scatter light in spectrophotometric measurements, leading to falsely elevated ammonia readings.

To mitigate lipemia interference:

• Advise patients to fast before phlebotomy if possible.
• Use special techniques or equipment to clear lipids from the sample before analysis.

anticoagulant

Anticoagulants are chemicals added to blood samples to prevent clotting. For serum ammonia measurements, using the correct anticoagulant is essential.

The preferred anticoagulant for ammonia measurement is EDTA because it effectively prevents clotting without affecting ammonia levels. Using a red clot tube without anticoagulant is incorrect, as it allows blood to clot, which can degrade ammonia.

Always ensure to use the right type of tube to avoid preanalytical errors.