Frequently Asked Questions – Clinical Chemistry Reagents

Liquid N-geneous^® HDL

Q. My controls have shifted. What do I do?

A. Check to see if the lot number on the controls have changed, especially checking that the patient results have not been affected. If patient results are not affected, then controls may need to be adjusted. If the calibrator lot has been changed recently, then check to make sure the target value is correct. Also if a new lot of reagent has been put into use and patient results are not affected, then control ranges may need to be adjusted

Q. I am seeing a bias (negative or positive) with my CAP proficiency survey results. What can I do?

A. Check to be sure the user is reporting under method code 225: Liquid Selective Detergent. This may be a reason for the bias seen.

Q. I do not believe the HDL result I got in my testing. What should I do?

A. First, repeat the original serum or plasma sample testing. Also, the user should look to see if the other lipid results run seem to be erroneous. The user should check to be sure the serum or plasma sample had been stored at 2-8 degrees C if not analyzed promptly.

Also, the user may want to see if citrated plasma was run. Citrated plasma is not acceptable; use edta, sodium, or lithium. If possible, recollect the specimen and repeat.

N-geneous^® LDL

Q. I have heard that the Friedewald Formula isn’t always accurate. Is this true?

A. Currently, some clinical laboratories utilize the equation known as the Friedewald Formula to estimate a patient’s LDL cholesterol concentration.¹ The formal uses the following calculation:

The formula estimates the LDL cholesterol concentration by subtracting the cholesterol associated with the other classes of lipoproteins from total cholesterol. This involves three independent lipid analyses, each contributing a potential source of error. It also involves a potentially inaccurate estimate of VLDL cholesterol. Since, no direct VLDL cholesterol assay is available, it is calculated from the triglyceride value divided by a factor of 5 (triglyceride/2.2 for mmol/L). This divisor can also add error to all LDL cholesterol estimates, but is especially inappropriate for individuals with elevated triglyceride levels.

The drawbacks of using the Friedewald Formula for determining levels of LDL cholesterol are: it is estimated by calculation; it requires multiple assays and multiple steps each adding a potential source of error; it is increasingly inaccurate as triglyceride levels increase; it requires that patients fast for 12 to 14 hours prior to specimen collection to avoid triglyceride bias; and, it is not standardized.^1,2

¹ Friedewald, WT, et al., Estimation of the Concentrations of Low-Density Lipoprotein Cholesterol in Plasma, Without Use of the Preparative Ultracentrifuge, CLIN.CHEM., 18/6:499 (1972).

² Rifai, Nader, et al., Measurement of Low-Density-Lipoprotein Cholesterol in Serum: a Status Report, CLIN. CHEM. 38/1: 150 (1992).

Q. On a recent proficiency survey my peer comparison looks ok, however the comparison to the reference method (ultracentrifugation) has a significant bias. Do you know what this is caused from?

A. Matrix effects are not uncommon with proficiency material, especially with lipids. Results should be evaluated against the peer group for LDL. Comparison of the homogeneous LDL to ultracentrifugation should be performed using fresh serum samples.

Q. My controls have shifted. What do I do?

A. Check to see if the lot number on the controls have changed, especially checking that the patient results have not been affected. If patient results are not affected, controls may need to be adjusted. If the calibrator lot has been changed recently, check to make sure the target value is correct. Also if a new lot of reagent has been put into use and patient results are not affected, control ranges may need to be adjusted.