lipaemia

Lipaemia (also Lipemia) describes an abnormally high concentration of lipids in the blood, usually in the form of very low density lipoproteins (VLDLs) or chylomicrons. Characteristically the blood plasma may appear white or milky in colour due to the presence of fat.

Triglycerides in the 400–800 mg/dl range may produce visible lipaemia.

Causes of lipaemia

Common causes of lipaemia are:

• Eating fatty foods. This may happen soon after a fatty meal, although patients who are very overweight may constantly have mildly lipaemic blood
• Diabetes mellitus
• Alcoholism
• Medications such as steroids, oestrogens, and protease inhibitors (for HIV infection)
• Chronic renal failure
• Hypothyroidism
• Pancreatitis and other pancreatic or hepatic disorders
• Multiple myeloma
• Primary biliary cirrhosis
• Systemic lupus erythematosus
• Total parenteral nutrition

Interference with laboratory testing

Assays and analysers routinely used for biochemical and haematological laboratory tests can be classified according to how they work:

• Assays involving light scattering (these are affected by lipaemia)
• Assays involving volume displacement (these are affected by lipaemia)
• Assays involving the aqueous fraction (these are not affected by lipaemia)
• Assays involving optical clot detection methods (these are affected by lipaemia)
• Mechanical or electromechanical means of clot detection (these are not affected by lipaemia)

In lipaemia, chylomicrons and VLDLs are suspended in the blood and scatter light, producing the characteristic cloudiness or turbidity similar to that seen in milk. This suspension interferes with laboratory instrument systems that function based on light detection or scatter (turbidimetry and nephelometry).

The lipid particles also exert a volume effect and can interfere with laboratory assays involving volume displacement. This effect can cause a pseudo-hyponatremia for example.

Assays involving the aqueous fraction, such as methods involving ionselective electrodes are not affected by lipaemia.

Using optical clot detection methods, as is commonly used in determining prothrombin time and activated partial thromboplastin time, lipemia may result in artificial prolongation of clotting times.

Mechanical or electromechanical means of clot detection are not affected by lipemia.

Tests affected by lipaemia

Lipemia artificially increases values of the following analytes:

• Glucose
• Phosphorus
• Bilirubin
• Uric acid
• Total protein
• Hb_A1c
• Fructosamine
• Triglycerides
• D-dimer

Lipaemia artificially decreases values of the following analytes:

• Sodium (pseudohyponatremia)
• HDL cholesterol
• Ceruloplasmin
• Prealbumin
• Transferrin

Dealing with a lipaemic sample

Many instruments employ an optical detection method to measure the optical density of a sample. If a sample is lipaemic and the baseline optical density is too high, the instrument will not report a result. Other techniques will be needed to analyse the sample:

• Ask the patient to fast for 12 hours before sample collection
• Dilution. Interference by lipaemia is dependent on the dilution of samples used in the reaction. The higher the dilution that can be afforded, the lesser the chance of lipid-based interference. For example, samples diluted 1:20 can show lipid interference, which may be avoided using dilutions such as 1:400.
• Ultracentrifugation can separate out a fatty layer which is discarded before normal analysis
• Solvent extraction. Extraction of lipids from blood specimens with n-hexane before coagulation testing has been described but is not common practice in clinical laboratories

References

• Kroll MH, Elin RJ, "Interference with clinical laboratory analyses", Clinical Chemistry 1994 (40), 1996-2005

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