Lipids methodology

lipid profile tests

1. LIPIDS: METHODOLOGY LIPID PROFILE: CHOLESTEROL (HDL-C & LDL-C), TRIGLYCERIDES AND APOLIPOPROTEINS By Dr . BASIL, B – MBBS (Nigeria) , Department of Chemical Pathology/Metabolic Medicine, Benue State University Teaching Hospital, Makurdi . March, 2015

2. OUTLINE: Introduction Pre-analytical Considerations Analytical Methodology Cholesterol Lipoproteins Triglycerides Apolipoproteins Quality Control

3. INTRODUCTION Disorders of lipids is of immense importance to medical practice owing to its strong relations to Arteriosclerosis and thus obesity, HTN, DM and other abnormalities Good prognosis in the mgt of these conditions are predicated on early detection of deranged blood lipid profile Indications for Lipid Profile include: Screening for primary & secondary hyperlipidemias Monitoring for risk of atherosclerosis Monitoring treatment of hyperlipidemias CHOLESTEROL and TRIGLYCERIDE are the plasma lipids of most interest in the diagnosis and management of lipoprotein disorders

4. PREANALYTICAL CONSIDERATIONS It is important to standardize conditions under which blood specimen are drawn & prepared for analysis. Biological Variations: Age : cholesterol levels increase with age. Sex : women have lower level than men except in childhood & after early 50’s. Season : cholesterol levels are slightly higher in cold periods. Food intake: daily intake of fat increases cholesterol levels. Patients should be on their usual diet for 2wks and are neither gaining nor losing weight . Medical conditions: thyroid, liver, and kidney diseases Acute illness : It is recommended that lipoproteins measurement should be made no sooner than 8weeks after any form of trauma or acute bacterial/viral infection and 3 – 4 months after child birth Life-style : higher in sedentary and poor diet habits

5. Specimen Collection and Storage: P atient should fast for 12hours before sampling. The concentration of LDL-C/HDL-C declines after eating Chylomicrons are cleared within 6–9hrs and their presence after 12hrs fast is abnormal. Posture: Decreases of as much 10% in conc. of TC , LDL-C, HDL-C and apo -A-I and B, have been observed after 20 minutes recumbence patient to be seated for 5min prior to sampling to prevent hemo -concentration. Prolonged venous occlusion leads to increase in cholesterol conc by 10–15% Torniquet should not be for more than minutes or two . Exercise: Mild exercise produces a slight decrease in conc of cholesterol and TG that may persists for several days. Those who walk for about 4 hours each week have an average cholesterol conc. 5% lower and HDL-C conc 3.4% higher than inactive persons

6. Menstrual cycle: The plasma chol and TG conc tend to be highest at midcycle , the time of maximum estrogen secretion The cyclical variation in cholesterol is not observed with anovulatory cycles Diet: A high fat diet increases serum TGs. Ingestion of monounsaturated fat reduces cholesterol. Plasma triglyceride conc is reduced when sucrose intake is reduced. A high carbohydrate diet decreases the serum conc. of VLDL-C , TG, cholesterol and protein. Individuals who eat many small meals throughout the day tend to have conc. of total LDL and HDL-C that are lower than when same type and amount of food is eaten in three meals. Large protein meals at lunch or in the evening also increase the serum cholesterol for atleast 1 hour after a meal. In vegeterian individuals, conc. Of LDL & VLDL-C are reduced by 37 % and 12%.

7. Smoking: The plasma cholesterol , triglyceride and LDL cholesterol conc. are higher by about 3 %, 9.1 % and 1.7 % respectively in smokers than in non smokers. HDL cholesterol is lower in smokers than in non smokers Alcohol ingestion: When moderate amount of alcohol is ingested for 1wk , the serum TG conc is increased by more than 20mg/ dL . Prolonged moderate ingestion may increase HDL-C conc , which is associated with reduced plasma conc of cholesterol ester transfer protein(CETP). Plasma vs Serum: Either can be used when TC , TG and HDL-C are measured. Plasma is preferred when lipoprotein are measured by ultra-centrifugation and electrophoretic methods. Serum can be used when it is necessary to store samples for weeks or months.

8. Venous vs Capillary samples: Measurements in the capillary samples seem to be little lower than venous samples. Anticoagulants : Some anticoagulants such as citrate exert large osmotic effect resulting in falsely low plasma lipid and lipoprotein concentration. Heparin : because of its high M.W can alter electrophoretic mobility of lipoproteins. EDTA is preferred anticoagulant even though TC and TG conc i n EDTA plasma are 3 % lower than in serum. EDTA inhibits qxidation of lipids and proteolysis of apolipoprotein . TC values of EDTA plasma should be multiplied by 1.03 to make it comparable to serum values. Storage: TC, TG, HDL-C can be satisfactorily analyzed in frozen samples. Apolipoproteins can also be measured in frozen samples. Serum or plasma must be stored at – 70 o C if stored for long time. For short time storage ( upto a month or two ) the sample can be kept at – 2 o .

9. ANALYTICAL METHODOLOGY TOTAL CHOLESTEROL: Chemical Method: via Liebermann- Burchardt Reaction Modified Abell Kendall Method Bloors Method Enzymatic Method: Cholesterol oxidase (Routine Lab) GC–MS Method (Reference Method) Isotope Dilution Mass Spectrometry (Definitive Method) TRIGLYCERIDE: Chemical Method Van Handel and Zilversmith Enzymatic Method Glycerol Kinase GC–MS Method (Reference Method ) LIPOPROTEINS: Polyanion Precipitation Electrophoresis Ultra-centrifugation Immunochemistry OTHER METHODS: Mass Spectrometry Chromatography Homogenous assay

10. Cholesterol Estimation CHEMICAL METHODS: Abell Kendall Method (Former Reference Method): Principle: 3 step Cholesterol is hydrolyzed with alcoholic KOH Unesterified cholesterol is extracted with petroleum jelly Measured using the L-B Reaction Liebermann- Burchardt Reaction (L-B Reaction): Cholesterol + Sulfuric acid + A cetic anhydride => bluish green solution Bloors Method: Principle: 2 step Cholesterol is extracted using an alcohol ether mixture Measured using the L-B Reaction

11. ENZYMATIC METHOD: Cholesterol Oxidase Method (Routine Lab – Assay of Choice): Principle: Cholesterol ester + H 2 cholesterol esterase - > Free cholesterol Free Cholesterol cholesterol oxidase -> 4 cholestene-3-one + H 2 2 Trinders Reaction: H 2 2 + 4-aminophenazone peroxidase -> Quinoneimine dye (red) + H 2 O Read at 500nm wavelength Linear up to 600 – 700mg/ dL (15.54 – 18.13mmol/L ) Advantages (in comparison to the Chemical Method): Precise and accurate Lesser interferences – bilirubin, ascorbic acid, Hb Smaller sample quantity Rapid; does not require preliminary extraction step Can be used to measure unesterified cholesterol by omitting de-esterification step Mild reagents; better suited for automated analysers

12. Disadvantages: They are not absolutely specific for cholesterol. Cholesterol oxidase reacts with other sterols e.g plant sterol Ascorbic acid and Bilirubin interfere by consuming H 2 2 Bilirubin interference can produce falsely high or low values Significant only at conc >5mg/ dL decreasing Chol values by 5 – 15% GC–MS METHOD (Reference Method ): Specifically measures cholesterol and does not detect related sterols Shows good agreement with the Definitive Method - Isotope Dilution Mass Spectrometry Cholesterol Desirable level: < 200mg/ dL (< 5.2mmol/L) ; Conversion factor = 0.026

13. Estimation of Lipoproteins POLYANION PRECIPITATION: Lipoproteins are precipitated with polyanions ( heparin sulfate, dextran sulfate and phosphotungstate ) Reaction should be in the presence of divalent cations Mg, Ca and Mn M ost commonly for HDL and is reasonably specific LIPOPROTEIN ELECTROPHORESIS: Used to identify rare familial disorders ( e.g Type I, III, V Hyperlipidemia) Indications : serum TG > 300 mg/ dL fasting serum is lipemic significant hyperglycemia, impaired glucose intolerance serum uric acid > 8.5 mg/ dL clinical evidence of CHD or atherosclerosis in patient < 40 years of age .

14. Provides visual display useful in detecting unusual or variant patterns Agarose gel – most common; provides a clear background and convenient in use While PAGE gives a more detailed separation – fractionate LDL subclasses More useful in qualitative analysis Not desirable in LP quantitation due to poor precision and large systemic biases ULTRA-CENTRIFUGATION : Preparative Ultracentrifugation Uses sequential density adjustments of serum to fractionate major and minor classes of LP Density gradient methods (non-equilibrium or equilibrium techniques) permits fractionation or several or all classes of LPs in a single run

15. Workhorse for separation of LPs for quantitative purposes and preparative isolations Used in the reference methods for LP quantitation because LPs are classically defined in terms of hydrated density Tedious, expensive and technically demanding IMMUNOCHEMICAL METHODS: Use antibody-coated plates specific for epitopes on apolipoproteins both in routine and research lab

16. HDL–C Estimation : PRECIPITATION METHOD: Precipitating reagents such as divalent cations and polyanions are used to remove all lipoproteins except HDL Enzymatic method for total cholesterol ( Cholesterol Oxidase ) is used to quantitate HDL–C Demerit: Interference from elevated TG levels causing incomplete sedimentation after centrifuging which results in over estimation of HDL-C MAGNETIC METHOD Similar to the HDL-C precipitation method but uses a precipitant that is complexed to magnetic particle This sediments and does not require centrifugation Has been adapted for use in automated clinical chemistry analysers because, It allows the supernatant to be analysed without the need to remove it from the sedimented complex .

17. HOMOGENOUS ASSAY (Direct HDL-C Assay): Enzymatic method: First reagent – “blocks” non-HDLs Use of Antibodies or Polymers or complexing agent e.g Cyclodextrin Modification of cholesterol esterase and oxidase enzymes which makes them selective for HDL-C Use of blanking step that selectively consumes cholesterol from non-HDL species Second reagent – quantifies accessible HDL-C Highly precise and reasonably accurate but lacks specificity for HDL in unusual specimens e.g liver or kidney disease Does not require pretreatment The “Three-step Procedure”(Reference method for HDL-C estimation): Ultracentrifugation to remove VLDL Heparin manganese precipitation to remove LDL Analysis of supernatant cholesterol by the Abell Kendall assay It is tedious and expensive

18. LDL–C Estimation INDIRECT METHODS: Fridewald Equation(Calculation Method ) – Routine. LDL– Chol ( mmol /L) = [TC – HDL- Chol ] – Plasma TG/2.175; or, LDL- Chol (mg/ dL ) = [TC – HDL- Chol ] – Plasma TG/5 VLDL (mg/ dL ) = [TAG]/5 or VLDL ( mmol /L) = [TAG]/2.175 The factor [TAG]/5 is an estimate of the VLDL cholesterol and is based on the average ratio of triglyceride to cholesterol in VLDL Equation assumes patient fasted and plasma [TAG] does not exceed 5.0mmol/L Limitations: not appropriate in Samples with TG > 400mg/ dL Patients with suspected Dysbetalipoproteinaemia Other limitations: Does not account for cholesterol assosciated with IDL and Lp (a) Underestimate LDL-C in chronic alcoholics Unsuitable for monitoring Mis -classifies 15 – 40 % of patients when TG levels are between 200 to 400 mg/ dL

19. Beta-Quantification (Reference method): Tedious – reserved for samples where Fridewald equation is inappropriate 2 steps Ultracentrifugation to remove VLDL leaving behind LDL and HDL as well as IDL and Lp (a) Chemical Precipitation of HDL-C from either the whole serum or the infranate obtained from the ultracentrifugation LDL-C is calculated as difference btw Cholesterol measured in infranate and in the HDL fraction VLDL-C is usually calculated as the difference btw that in whole serum and the amount in the infranate fraction VLDL-C/Plasma TG ratio: may be useful in evaluation of type III hyperlipoproteinemia Expressed in mol / mol or mass/mass Ranges 0.230-0.575 in samples without beta VLDL Type III subjects have ratio > 0.689, usually in range of 0.689 – 0.0919

20. DIRECT METHODS: Methods based on selective Precipitation: Uses polyvinyl sulfate or heparin at low pH, OR Pretreatment using a mixture of polyclonal antibodies to apo A-1 and apo E linked to a resin to bind and remove VLDL, IDL, and HDL Homogenous method: Selectively measures LDL after masking non-LDL cholesterol, OR By selectively solubilizing LDL Advantages: Does not involve measurement of TGs thus non-fasting samples can be used Comparable to calculated LDL results from beta-quantitation on normolipemic specimens

21. Triaglycerides : CHEMICAL METHOD: First, Lipids are extracted using chloroform and phospholipids and removed by zeolite absorption Van Handel and Zilversmith Method (former Reference Method): Principle: TAG alcoholic KOH - > Glycerol + Fatty acids Glycerol + periodic acid ---------------> Formaldehyde Formaldehyde + Chromotropic acid-------------> Blue solution Demerit: Tedious, poorly charaterized GC–MS METHOD (Reference Method ): Hydrolysis of fatty acids on TGs and measurement of Glycerol NB: Accuracy in TG is less relevant than that for Chol , due to very large physiologic variation with CV of 25 – 30%, thus, contribution of analytical variation is insignificant

22. ENZYMATIC METHOD: Glycerol Kinase Method: Principle: TAG + 3H 2 lipase -> Glycerol + 3fatty acids Glycerol + ATP glycerol kinase -> Glycero phosphate + ADP Glycerophosphate Glycerophosphate Oxidase -> Dihydroxyacetone + H 2 O 2 Trinder’s reaction: H 2 O 2 + Chromogen peroxidase -> Pink compound + H 2 O Read absorbance at 500nm wavelength, and linear up to 700mg/ dL Merits: Fairly specific

23. Demerits: Glycerol kinase reacts with endogenous free glycerol causing interference which is clinically insignificant except in DM, emotional stress, IV admin of glycerol containing drugs or nutrients, contamination of blood collecting devices, and prolonged storage under non- refridgerated conditions Can be corrected for by “double-cuvette blank” or the “single-cuvette blank” or by “Designated calibration blanking” Or by enzymatically consuming glycerol in a prereaction step before measuring TG Triglycerides Desirable Level: <176 mg/dl (<2.0mmol/L); Conversion factor: 0.011

24. Apolipoproteins Apolipoproteins of clinical importance are: Apo B: an indicator of combined LDL and VLDL concentration Apo A-1: major protein of HDL Lp (a): the variant of LDL, an independent indicator of CHD risk Commonly measured by Immunoassays of different types IMMUNOTURBIDIMETRY: Most common method; advantages: Easily adapted spectrophotometric analysers allows the use of commercially available antisera and reference sera . Immunonephelometry can also be employed but requires a Nephelometer and so not commonly used These light scattering assays are subject to interferences from larger TG-rich lipoproteins and VLDL Other immunochemical methods available include: Enzyme-linked I mmunosorbent Assay (ELISA) Radial Immunodiffusion (RID) Radioimmunoassay (RIA) Antibodies used may be either monoclonal or polyclonal

25. QUALITY CONTROL Use of commercial control materials or Produced in-house from freshly collected patient serum, aliquoted into vials, quick frozen and stored at -70 o C Less subject to matrix interection compared to commercial ones Atleast two pools should be analysed with levels near decision points for each analyte .

26. REFERENCES Tietz textbook of Clinical Chemistry and Molecular Diagnostics; fifth ed., by Burtis et al. Clinical Chemistry; Principles, Techniques and Correlations, 7th ed., by Bishop et al . HENRY’S clinical diagnosis & management by lab. methods 21/e

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