Differences in reaction specificity toward lipoprotein X and abnormal LDL among 6 homogeneous assays for LDL-cholesterol

Use of therapeutic plasma exchange to remove lipoprotein X in a patient with vanishing bile duct syndrome presenting with cholestasis, pseudohyponatremia, and hypercholesterolemia: A case report and review of literature

INTRODUCTION

Lipoprotein X (Lp-X) is an abnormal lipoprotein found in multiple disease conditions, including liver dysfunction and cholestasis. High Lp-X concentrations can interfere with some laboratory testing that may result in spurious results. The detection of Lp-X can be challenging, and there is currently a lack of consensus regarding the management of Lp-X other than treating the underlying disease.

CASE PRESENTATION

A 42-year-old female with Hodgkin's lymphoma treated with dexamethasone, high dose cytarabine and cisplatin and vanishing bile duct syndrome confirmed by liver biopsy presented with cholestasis, pseudohyponatremia (sodium, 113 mmol/L; reference range 136-146 mmL/L; serum osmolality, 303 mOsm/kg), and hypercholesterolemia (> 2800 mg/dL, reference range < 200 mg/dL). Lp-X was confirmed by lipoprotein electrophoresis (EP). Although she did not manifest any specific signs or symptoms, therapeutic plasma exchange (TPE) was initiated based on laboratory findings of extreme hypercholesterolemia, spuriously abnormal serum sodium, and HDL values, and the potential for short- and long-term sequelae such as hyperviscosity syndrome, xanthoma, and neuropathy. During the hospitalization, she was treated with four 1.0 plasma volume TPE over 6 days using 5% albumin for replacement fluid. After the first TPE, total cholesterol (TC) decreased to 383 mg/dL and sodium was measured at 131 mmol/L. The patient was transitioned into outpatient maintenance TPE to eliminate the potential of Lp-X reappearance while the underlying disease was treated. Serial follow-up laboratory testing with lipoprotein EP showed the disappearance of Lp-X after nine TPEs over a 10-week period.

LITERATURE REVIEW

There are seven and four case reports of Lp-X treated with TPE and lipoprotein apheresis (LA), respectively. While all previous case reports showed a reduction in TC levels, none had monitored the disappearance of Lp-X after completing a course of therapeutic apheresis.

CONCLUSION

Clinicians should have a heightened suspicion for the presence of abnormal Lp-X in patients with cholestasis, hypercholesterolemia, and pseudohyponatremia. Once Lp-X is confirmed by lipoprotein EP, TPE should be initiated to reduce TC level and remove abnormal Lp-X. Most LA techniques are not expected to be beneficial since Lp-X lacks apolipoprotein B. Therefore, we suggest that inpatient course of TPE be performed every other day until serum sodium, TC and HDL levels become normalized. Outpatient maintenance TPE may also be considered to keep Lp-X levels low while the underlying disease is treated. Serum sodium, TC, and HDL levels should be monitored while on maintenance TPE.

A High-Throughput NMR Method for Lipoprotein-X Quantification

Lipoprotein X (LP-X) is an abnormal cholesterol-rich lipoprotein particle that accumulates in patients with cholestatic liver disease and familial lecithin-cholesterol acyltransferase deficiency (FLD). Because there are no high-throughput diagnostic tests for its detection, a proton nuclear magnetic resonance (NMR) spectroscopy-based method was developed for use on a clinical NMR analyzer commonly used for the quantification of lipoproteins and other cardiovascular biomarkers. The LP-X assay was linear from 89 to 1615 mg/dL (cholesterol units) and had a functional sensitivity of 44 mg/dL. The intra-assay coefficient of variation (CV) varied between 1.8 and 11.8%, depending on the value of LP-X, whereas the inter-assay CV varied between 1.5 and 15.4%. The assay showed no interference with bilirubin levels up to 317 mg/dL and was also unaffected by hemolysis for hemoglobin values up to 216 mg/dL. Samples were stable when stored for up to 6 days at 4 °C but were not stable when frozen. In a large general population cohort (n = 277,000), LP-X was detected in only 50 subjects. The majority of LP-X positive cases had liver disease (64%), and in seven cases, had genetic FLD (14%). In summary, we describe a new NMR-based assay for LP-X, which can be readily implemented for routine clinical laboratory testing.

The necessity for improving lipid testing reagents: A real world study

BACKGROUND

We investigated the interference of vitamin C (VitC), glycerol fructose, lipoprotein X (LpX) and lipemia on the analysis of serum lipids.

METHODS

Serum were collected from 44 patients with VitC infusion, serum lipid concentrations before and after VitC auto-oxidation were compared. Serum of 31 patients with glycerol fructose infusion were collected, triglycerides (TG) measured by glycerol blanking and non-blanking reagents were compared. Forty-four serum samples suspected to contain LpX were collected, LDL-C measured by reagents from five manufacturers were compared. Lipemia samples were collected, LDL-C measured using five different reagents were compared. The interference rate was considered unacceptable if it was greater than 1/2 total allowable error (TEa).

RESULTS

In patients with VitC infusion, the interference rates of TG and total cholesterol (TC) were -59% (-123%, -28%) and -15% (-21%, -11%), respectively. In patients with glycerol fructose infusion, the interference rate of TG was 13% (4%, 113%). LpX interference led to increased LDL-C results for most reagents. Lipemia caused great interference with LDL-C analysis.

CONCLUSION

VitC, glycerol fructose, LpX and lipemia significantly interfered with lipid assays. The reagent formulation should be improved to get reliable results.

Clinical laboratory characteristics of patients with obstructive jaundice accompanied by dyslipidemia

BACKGROUND

Abnormal lipid metabolism manifests as hypercholesterolemia in patients with obstructive jaundice due to lipoprotein X (LpX). Our aim was to explore the clinical laboratory characteristics of patients with obstructive jaundice accompanied by dyslipidemia in a large number of samples.

METHODS

A total of 665 patients with obstructive jaundice were included and categorized into two groups (with/without dyslipidemia) based on the ratio of the sum of HDL-c and LDL-c to total cholesterol [(HDL-c + LDL-c)/TC] with a cut-off value of 0.695. Laboratory liver, kidney, and blood lipid parameters were determined. Cholesterol composition assessment was performed by ultracentrifugation and high-performance liquid chromatography (UC-HPLC), and serum protein profiles were analyzed by capillary electrophoresis.

RESULTS

Liver function in patients with obstructive jaundice accompanied by dyslipidemia was more aggravated than that in patients with simple obstructive jaundice (P < 0.05). The (HDL-c + LDL-c)/TC ratio was negatively correlated with bilirubin levels (P < 0.05). In addition, the difference in ApoB/LDL-c ratios was statistically significant between the obstructive jaundice accompanied by dyslipidemia group and healthy control group (P < 0.05). The LDL-c concentration determined by the UC-HPLC method was more than five times that determined by the enzymatic method (P < 0.05). Bisalbuminemia was found in 43 of 60 patients with obstructive jaundice accompanied by hypercholesterolemia.

CONCLUSIONS

In patients with obstructive jaundice, the decreased (HDL-c + LDL-c)/TC ratio may be a novel marker to identify dyslipidemia secondary to LpX. The decreased ratio was associated with poor liver function and indicated disease progression.

Comparison of Two Homogeneous LDL-Cholesterol Assays Using Fresh Hypertriglyceridemic Serum and Quantitative Ultracentrifugation Fractions

Aim: The purpose of this study was to compare two homogeneous assays of low-density lipoprotein-cholesterol (LDL-C) with a modified beta quantification reference measurement for LDL-C (BQ-LDL), fractions of chylomicron (CM), very low-density lipoprotein (VLDL) and intermediate-density lipoprotein (IDL) by quantitative ultracentrifugation in patients with hypertriglyceridemia.

Methods: Two homogeneous LDL-C assays (LDL-C(K), Kyowa Medex and LDL-C(S), Sekisui Medical) were used to measure 198 samples of fresh anonymized leftover sera with hypertriglyceridemia (≥ 150 mg/dL). Of these, 32 samples with discrepant LDL-C levels or hypertriglyceridemia (≥ 400 mg/dL) were used for further analysis. Quantitative ultracentrifugation was used to separate samples.

Results: The two homogeneous LDL-C assays had a strong correlation with each other for the samples from 198 patients with hypertriglyceridemia. LDL-C(K) and LDL-C(S) in 32 selected samples were strongly correlated with BQ-LDL. In both homogeneous assays, cholesterol in the CM and VLDL fractions was measured as part of the LDL-C. A weak correlation was found between cholesterol in the VLDL fraction and LDL-C using the two homogeneous assays, but no correlation was found with cholesterol in the CM fraction. Cholesterol in the IDL fraction was also measured as part of the LDL-C in both assays.

Conclusion: Both homogeneous assays partially detected cholesterol in the chylomicron and VLDL fractions, but LDL-C measured by both homogeneous assays correlated with BQ-LDL.

Detection of Lipoprotein X (LpX): A challenge in patients with severe hypercholesterolaemia

Background

Lipoprotein X (LpX) is an abnormal lipoprotein fraction, which can be detected in patients with severe hypercholesterolaemia and cholestatic liver disease. LpX is composed largely of phospholipid and free cholesterol, with small amounts of triglyceride, cholesteryl ester and protein. There are no widely available methods for direct measurement of LpX in routine laboratory practice. We present the heterogeneity of clinical and laboratory manifestations of the presence of LpX, a phenomenon which hinders LpX detection.

Methods

The study was conducted on a 26-year-old female after liver transplantation (LTx) with severely elevated total cholesterol (TC) of 38 mmol/L and increased cholestatic liver enzymes. TC, free cholesterol (FC), cholesteryl esters (CE), triglycerides, phospholipids, HDL-C, LDL-C, and apolipoproteins AI and B were measured. TC/apoB and FC:CE ratios were calculated. Lipoprotein electrophoresis was performed using a commercially available kit and laboratory-prepared agarose gel.

Results

Commercially available electrophoresis failed to demonstrate the presence of LpX. Laboratory-prepared gel clearly revealed the presence of lipoproteins with γ mobility, characteristic of LpX. The TC/apoB ratio was elevated and the CE level was reduced, confirming the presence of LpX. Regular lipoprotein apheresis was applied as the method of choice in LpX disease and a bridge to reLTx due to chronic liver insufficiency.

Conclusions

The detection of LpX is crucial as it may influence the method of treatment. As routinely available biochemical laboratory tests do not always indicate the presence of LpX, in severe hypercholesterolaemia with cholestasis, any discrepancy between electrophoresis and biochemical tests should raise suspicions of LpX disease.

Controversy over the atherogenicity of lipoprotein-X

PURPOSE OF REVIEW

Lipoprotein-X (Lp-X) is an abnormal lipoprotein containing abundant free cholesterol and phospholipids, as well as some apolipoprotein E (apoE). Serum Lp-X increases in patients with cholestasis and lecithin-cholesterol acyltransferase deficiency, as well as in those receiving intravenous lipid emulsion. Lp-X is often associated with skin xanthomas in cholestatic patients. However, earlier studies showed that Lp-X is not taken up by murine macrophages, but exerts antiatherogenic actions. In this review, we discuss the heterogeneity of Lp-X and its potential atherogenicity.

RECENT FINDINGS

Mass spectrometry revealed that Lp-X of cholestatic patients is similar in lipid composition to low-density lipoprotein (LDL) and high-density lipoprotein, but not to bile acids, suggesting that Lp-X is synthesized in the liver. Palmar xanthomas appear in patients with cholestasis, but regress over months after improvement of hypercholesterolemia. Lp-X isolated from cholestatic patients is rich in apoE, and causes more lipid accumulation than oxidized LDL and acetyl LDL in human monocyte-derived macrophages.

SUMMARY

Lp-X is heterogeneous in apoE content. Lp-X is taken up in cholestatic patients by apoE-recognizing lipoprotein receptors. Further research is warranted to fully understand the atherogenicity of Lp-X and the clinical significance of elevated Lp-X concentrations, particularly in cholestatic patients.

Homogeneous Assays for LDL-C and HDL-C are Reliable in Both the Postprandial and Fasting State

AIM

Most epidemiological and clinical studies calculated low-density lipoprotein-cholesterol (LDL-C) by Friedewald's formula which cannot be used in the postprandial samples. Although the homogeneous assays with poor analytical performance were withdrawn from the market, it remained unclear whether the currently available reagents for LDL-C and high-density lipoprotein-cholesterol (HDL-C) are as accurate for postprandial samples as for fasting samples.

METHODS

Fresh blood samples were collected from 59 non-diseased and 109 diseased subjects. Postprandial samples constituted 72.9% and 39.4% of these samples. LDL-C and HDL-C concentrations were measured using the homogeneous assays of four manufacturers (Denka Seiken, Wako, Kyowa Medex, and Sekisui Medical). Simultaneously, LDL-C and HDL-C concentrations were determined using the reference measurement procedures (RMPs) of the Centers for Disease Control and Prevention (CDC). Total errors were calculated using a routine method (TE_com) and via error component analysis (TE_ECA).

RESULTS

All homogeneous assays for LDL-C and HDL-C met the National Cholesterol Education Program (NCEP) requirements in terms of coefficient of variation, and TE_com in both non-diseased and diseased subjects. LDL-C and HDL-C values measured by the homogeneous assays were in good agreement with those measured by the RMPs in both fasting and postprandial samples. The TE_com and TE_ECA values of the postprandial samples were similar to those of fasting samples, although the TE_ECA values were up to 4.4-fold greater than the TE_com values.

CONCLUSIONS

In both non-diseased and diseased subjects, the homogeneous assays for LDL-C and HDL-C of four manufacturers are as accurate for postprandial samples as for fasting samples.

Lipid profiling of lipoprotein X: Implications for dyslipidemia in cholestasis

Lipoprotein X (Lp-X) is an abnormal lipoprotein that may typically be formed in intra- and extrahepatic cholestasis and potentially interfere with lipid analysis in the routine lab. To gain insight into lipid class and species composition, Lp-X, LDL and HDL from cholestatic and control serum samples were subjected to mass spectrometric analysis including phospholipids (PL), sphingolipids, free cholesterol (FC), cholesteryl esters (CE) and bile acids. Our analysis of Lp-X revealed a content of 46% FC, 49% PL with 34% phosphatidylcholine (PC) as main PL component. The lipid species pattern of Lp-X showed remarkable high fractions of mono-unsaturated species including PC 32:1 and PC 34:1 and phosphatidylethanolamine (PE) 32:1 and 34:1. LDL and HDL lipid composition in the same specimens strongly reflected the lipid composition of Lp-X with increased PC 32:1, PC 34:1, PE 32:1, PE 34:1 and FC accompanied by decreased CE compared to controls. Comparison of Lp-X and biliary lipid composition clearly indicates that Lp-X does not originate from a sole release of bile lipids. Moreover, these data present evidence for increased hepatic fatty acid and PL synthesis which may represent a reaction to high hepatic FC level observed during cholestasis.