In this excellent case study, the authors show the importance of looking deeper than just the lipid numbers initially returned from the laboratory. This case highlights the pitfalls for the unwary physician who fails to use all the available clinical information when making a decision as to what the measured lipid level means for the patient. The authors are to be commended for allowing us to share this excellent example of careful diagnostic evaluation. We hope it will be of great interest to the readership.
Laboratory studies at the time of her visit revealed an alkaline phosphatase of 1328 U/L (normal 39-100), aspartate aminotransferase (AST) 122 U/L (8-43), alanine aminotransferase (ALT) 80 U/L (7-45), and total bilirubin 3.0 mg/dl (0.1-1.0). Total cholesterol was 1060 mg/dl (optimal goal < 200), triglycerides 169 mg/dl (<150), HDL-C 24 mg/dl (>50), and LDL-C 1002 mg/dl (< 130). The marked elevations in total cholesterol and LDL-C prompted further laboratory evaluation with agarose lipoprotein electrophoresis and polyacrylamide gel lipoprotein electrophoresis. LDL-C appeared to be present on the agarose gel, but the dark, smearing nature of the gel with slight reverse migration was consistent with the presence of lipoprotein-X (LpX). This finding prompted additional testing using ultracentrifugation with β-quantification of LDL-C and HDL-C. Lipid measurements with ultracentrifugation method revealed a total cholesterol of 1060 mg/dl, triglycerides 169 mg/dl, HDL-C 201 mg/dl, LDL-C 740 mg/dl, and VLDL 119 mg/dl. Apolipoprotein-B (apoB) measurement was requested and was reported as 247 mg/dl (44-148). Lp(a) and HDL-C were not visible on the gel.
Homogeneous assays for LDL-C are known to be markedly affected by the presence of LpX in serum (3). In this case report, lipid analysis with ultracentrifugation gave substantially lower LDL-C and higher HDL-C concentration’s compared to the commonly used automated Roche-Hitachi instrument measurement. It is likely that LpX present in the bottom fraction during ultracentrifugation was not fully precipitated along with LDL and thus was measured as HDL-C. Because the interaction of LpX with other lipoproteins is not fully understood, assessment of LDL status in a patient with increased LpX is likely to yield inaccurate results. While blood samples with very high concentrations of LpX might interfere with the methods for apoB determination, apoB measurement could be used to better assess LDL status (1). In this case report, while lipoprotein electrophoresis analysis showed the presence of LpX, the patient also had elevated apoB levels (247 mg/dl). Based on this finding and the LDL serum migratory pattern on gel electrophoresis, LpX was estimated as approximately 50% of the total measured (740 mg/dl) LDL-C. Because of elevated apoB, she underwent further cardiac risk assessment and management. Coronary angiogram documented severe multivessel disease requiring placement of three multi-length bare metal stents. She was referred to the Nicotine Dependence Center to help her with smoking cessation.
Hypercholesterolemia is associated with increased risk of atherosclerosis and heart disease. However, hypercholesterolemia due to LpX is not believed to be atherogenic. The exact reason for this is unclear. The rich phospholipid and nonesterified cholesterol (poor cholesteryl ester), triglyceride, and protein content of LpX are all believed to play a role. In-vitro, LpX has been shown to inhibit oxidation of LDL and to protect endothelial cells by attenuating the degradation of fibroblast growth factors (4). Clinical data from patients with PBC and hyperlipidemia reported no clear increase in cardiovascular events despite LDL-C levels above those defined as optimal by the National Cholesterol Education Program Adult Treatment Panel III guidelines (5,6,7). It should be noted that the bulk of the data are observational, include studies with small sample sizes, and may not be powered adequately to detect differences in cardiovascular events (5,6,7).
Some investigators argue that prolonged bile stasis in PBC could worsen arterial disease (8). Yet, it is unclear if this effect would be due to LpX, per se, or to the concomitant elevation of apoB and/or other atherogenic particles in PBC patients with additional cardiac risk factors. In patients with PBC and cholestasis, high total cholesterol levels may not reflect a greater content of apoB and increased atherogenicity. In fact, it is not uncommon for these patients to have significantly elevated total cholesterol that is predominantly LpX. Treatment strategies should be guided by an assessment of the patients overall cardiac risk obtained by a thorough history and examination, and appropriate cardiac studies.
Statins are extensively metabolized by the liver through hydroxylation and beta-oxidation (cytochrome P450 CYP3A4), and do not undergo enterohepatic recirculation. Statins are also candidates for drug-drug interactions when co-administered with potent CYP3A4 inhibitors, such as itraconazole, erythromycin, and grapefruit juice. Glucuronidation is another step in the metabolism of statins and their metabolites, and gemfibrozil is believed to inhibit glucuronidation of the statins. Thus, in patients with PBC and compromised biliary and/or hepatic function whose LDL levels are confounded by LpX, the potential benefit(s) of using a statin, alone or in combination with other antihyperlipidemic agents, ought to be weighed against the increased risk of medication-related side effects. Initial therapy should address treating the underlying cause, i.e., cholestasis, and ursodeoxycholic acid is a commonly used therapeutic agent for the treatment of PBC. Studies using statins (9) or fibrates (10) in patients with PBC reported improvement in hepatic enzymes and lipid parameters, but did not address the effects of treatment on survival. However, treatment of hyperlipidemia is justified if cardiovascular risk factors are present (11) and/or non-LpX levels (i.e., apoB) are elevated. In the case discussed, documented severe coronary artery disease, intolerance to statin therapy, and elevated apoB concentrations prompted a referral for LDL-apheresis [using dextran sulfate cellulose adsorption (DSC) columns that bind LDL]. Significant improvement in LDL (8) and apoB (12) levels with the use LDL-apheresis has been reported in patients with PBC.
- LpX is a free (non-esterified) cholesterol and phospholipid-rich lipoprotein associated with cholestatic liver disease.
- LpX interferes with routine lipid assays and can result in inaccurate lipid measurements.
- Based on mostly observational clinical data and some basic science studies, LpX does not appear to be atherogenic.
- Initial therapy should be directed to treating the cholestasis.
- LpX may coexist with atherogenic-proven lipoprotein particles, i.e., apoB. In such cases, careful assessment of cardiac risk factors should guide therapeutic interventions. If indicated, statins and/or fibrates should be used with caution in patients with cholestasis. LDL-apheresis has been used to help achieve therapeutic targets for lipids.
- Foley KF, Silveira MG, Hornseth JM, Lindor KD, McConnell JP. A patient with primary biliary cirrhosis and elevated LDL cholesterol. Clin Chem. 2009;55(1):187-91.
- Narayanan S. Biochemistry and clinical relevance of lipoprotein X. Ann Clin Lab Sci 1984;14:371-374
- Fei H, Maeda S, Kirii H, Fujigaki S, Maekawa N, Fujii H, et al. Evaluation of two different homogeneous assays for LDL-cholesterol in lipoprotein-X-positive serum. Clin Chem 2000;46:1351-1356.
- Chang PY, Lu SC, Su TC, Chou SF, Huang WH, Morrisett JD, Chen CH, Liau CS, Lee YT. Lipoprotein-X reduces LDL atherogenicity in primary biliary cirrhosis by preventing LDL oxidation. J Lipid Res. 2004;45(11):2116-22.