We retrospectively enrolled 1,844 Korean individuals who had undergone coronary CT angiography using 64-slice MDCT and measurements of serum apoB and apoA1 concentrations during general routine health evaluations at the Asan Medical Center (AMC, Seoul, Korea) from January 2008 to December 2009. Each subject completed a questionnaire assessing medications, history of previous medical and/or surgical diseases, and drinking and smoking habits. Drinking habits were categorized as never or rarely, 1-3 times/week, 4-6 times/week, or almost every day; smoking habits were categorized as never, previous or current.

History of CVD was based on each subject's history of angina, myocardial infarction and/or cerebrovascular accidents. Subjects with diabetes were defined as those with fasting plasma glucose (FPG) levels of ≥ 126 mg/dL and/or taking antidiabetic medications. Hypertension was defined as systolic and/or diastolic blood pressures (BP) ≥ 140/90 mmHg or administration of antihypertensive medications. We excluded subjects with chest pain, a history of CVD or percutaneous coronary intervention, or prior coronary arterial bypass surgery (n = 91), as well as those who had taken drugs for more than 6 months or within the previous 12 months that could potentially affect lipid metabolism (n = 200). In addition, subjects with abnormal liver (aspartate aminotransferase, AST and/or alanine aminotransferase, ALT ≥ 2.5 × upper limit of normal value, n = 78), kidney (serum creatinine > 1.5 in men and > 1.4 in women, n = 5) and thyroid function (TSH < 0.4 or > 5.0 mU/L and/or free T4 < 10.3 or > 24.5 pM/L, n = 69) were excluded as liver disease and thyroid disease have been known to be associated with changes in apolipoproteins (7, 8), and decreased renal function could also affect the metabolism of apolipoproteins (9). After exclusion of ineligible subjects, 1,401 participants (877 men, mean age 53.8 ± 9.8 yr; and 524 women, mean age 53.9 ± 9.8) were deemed eligible and were included in this study.

Height and weight were measured while subjects were wearing light clothing without shoes. Body mass index (BMI) was calculated as weight in kilograms divided by the square of height in meters. Waist circumference (cm) was measured midway between the costal margin and the iliac crest at the end of a normal expiration. BP was measured on the right arm after a rest ≥ 5 min, using an automatic manometer with an appropriate cuff size.

After overnight fasting, early morning blood samples were drawn from the antecubital vein into vacuum tubes and subsequently analyzed by a central, certified laboratory at the AMC. Measurements included the concentrations of uric acid, fasting glucose, insulin, and high-sensitive C-reactive protein (hsCRP) and concentrations of lipid parameters, including apoB and apoA1.

Fasting total cholesterol, HDL-C, LDL-C, TG, and uric acid concentrations were measured by an enzymatic colorimetric method using a Toshiba 200FR Neo (Toshiba Medical System Co., Ltd., Tokyo, Japan). Serum apoB and apoA1 concentrations were measured by a turbidometric method using a Cobas Integra C-6000 analyzer (Roche Diagnostics, Basel, Switzerland), hsCRP concentrations using an immunoturbidimetric method (Toshiba), and FPG concentrations by an enzymatic colorimetric method using a Toshiba 200 FR autoanalyzer (Toshiba). Hemoglobin A1c (HbA1c) concentrations were determined by ionexchange high-performance liquid chromatography (Bio-Rad Laboratories, Inc., Hercules, CA, USA) and serum insulin concentrations by an immunoradiometric assay (TFB Co., Ltd, Tokyo, Japan). Homeostatic model assessment of insulin resistance (HOMA-IR) was calculated as the product of fasting serum insulin (microunits per milliliter) and fasting plasma glucose (millimoles) concentrations, divided by 22.5. All enzyme activities were measured at 37℃.

MDCT examinations were performed using a VCT XT 64-slice MDCT scanner (GE Healthcare, Milwaukee, WI, USA). Subjects with a heart rate > 70 beats/min were given beta-blocking agents before MDCT. A standard scanning protocol was used, with 64 × 0.624 mm slice collimation, 350 ms rotation time, 100-120 kVp tube voltage, and 500-800 mA tube current, according to subject body habitus. All scans were performed using electrocardiogram-gated dose modulation. A bolus of 80 mL iomeprol (Iomeron 400; Bracco, Milan, Italy) was intravenously injected (4 mL/s), followed by a saline flush of 50 mL.

A region of interest was defined on the ascending thoracic aorta, and image acquisition was automatically initiated once a selected threshold (120 Hounsfield units [HU]) was reached, with bolus tracking. Each subject's electrocardiogram was simultaneously recorded to allow for retrospective segmental data reconstruction. Images were initially reconstructed at middiastolic or end-systolic phase of the cardiac cycle. The average radiation dose for MDCT was 4.7 ± 1.6 mSv.

All data were evaluated on a remote workstation (Advantage Workstation; GE Healthcare). Each lesion was identified using a multiplanar reconstruction technique and the maximal intensity projection of short-axis, two-chamber, and four-chamber views. Coronary artery stenosis and plaque characteristics were analyzed on a segmental basis according to American Heart Association criteria (10).

The contrast-enhanced portion of the coronary lumen was semi-automatically traced at the maximal stenotic site and compared with the mean value of the proximal and distal reference sites. Stenosis > 50% was defined as significant.

Plaques were defined as structures > 1 mm² within and/or adjacent to the vessel lumen. Plaques consisting of calcified tissue occupying more than 50% of the plaque area (density > 130 HU in native scans) were classified as calcified, plaques with < 50% calcium were classified as mixed, and plaques without any calcium were classified as noncalcified lesions (11).

Coronary artery calcium scores (CACS) were measured as described previously (12). Participants, on the basis of the CACS, were categorized in the following manner: no, 0; mild, 0.1 to 100; moderate to severe, > 100.1.

Continuous variables with normal distribution were expressed as mean ± SD, whereas continuous variables with skewed distribution were expressed as median (and interquartile range). Categorical variables were expressed as proportions (%). The apoB/A1 quartiles were Q1 ≤ 0.47, Q2 = 0.48-0.60, Q3 = 0.61-0.74, and Q4 ≥ 0.75. Demographic and biochemical characteristics of the study population assorted according to apoB/A1 quartiles were compared using one-way analysis of variance (ANOVA) or the Kruskal-Wallis test for continuous variables and the chi-square test for categorical variables. The apoB/A1 ratios of subjects with stenosis and without significant stenosis were compared using the unpaired Student's t-test or the Mann-Whitney U-test.

Multivariate logistic regression analysis, after adjustment for confounding variables, was used to calculate the odds ratios (ORs) of apoB/A1 ratio for significant coronary stenosis or plaques. To assess the utility of apoB/A1 ratio as a marker for significant coronary stenosis, we constructed receiver operating characteristics (ROC) curves and calculated the areas under the curve (AUC). The distance on the ROC curve of apoB/A1 was calculated by plotting the sensitivity against (1-specificity). The AUC and the crucial points were determined using Med-Calc® version 11.6.1.0 for Windows (MedCalc Software, Mariakerke, Belgium). All other statistical analyses were performed using SPSS version 14.0 for Windows (SPSS Inc., Chicago, IL, USA). A P value < 0.05 was considered statistically significant.

This study was approved by the institutional review board of the Asan Medical Center (IRB No. 2011-0728). All subjects enrolled in this study provided written informed consent.

The clinical and biochemical characteristics of the study population classified according to apoB/A1 quartiles are presented in Table 1. We observed positive relationships between apoB/A1 quartiles and BMI, WC, SBP, DBP, FPG, uric acid, total cholesterol, TG, LDL-C, hsCRP, HOMA-IR, and percentages of male subjects and current smokers (P for trend < 0.001 each except for SBP), and a significant negative relationship between apoB/A1 quartiles and HDL-C (P for trend < 0.001). We observed no relationship between apoB/A1 quartiles and the percentages of subjects with diabetes and hypertension.

Significant coronary artery stenosis was detectable in 114 subjects (8.1%), 89 men and 25 women. An increase in apoB/A1 quartiles was associated with an increased percentage of subjects with significant coronary stenosis (P for trend = 0.003, Table 2). Of the 114 patients with significant stenosis, 87 (76.3%) had single-vessel disease. We also observed a positive relationship between apoB/A1 quartiles and increased percentage of subjects with plaques (P for trend = 0.001, Table 2). Among plaque subtypes, only noncalcified plaques (NCAP) differed significantly across the apoB/A1 quartiles (P for trend < 0.001, Table 2). Regarding to CACS, subjects with higher CACS were observed in those with higher apoB/A1 quartiles (P for trend = 0.001, Table 2)

Fig. 1 shows the distribution of apoB/A1 ratios according to the presence of significant coronary stenosis. We found that apoB/A1 ratios were significantly higher in subjects with than without stenosis.

Among the 114 subjects with significant stenosis detected by MDCT, 51 (44.7%, 41 men and 10 women) underwent conventional coronary angiography. Angiography indicated coronary stenosis consistent with the lesions detected by MDCT in 45 subjects (88.2%), but milder coronary stenosis than indicated by MDCT in 6 subjects (11.8%). Revascularization was performed in 31 subjects (68.9%), 28 (62.2%) by percutaneous coronary intervention and 3 (6.7%) by coronary artery bypass surgery.

Table 3 shows the relationship between apoB/A1 ratio and risk of significant coronary stenosis. For each 0.1 increment in apoB/A1 ratio, the ORs for significant stenosis, and any plaques in all subjects increased significantly, even after adjustment for confounding variables such as age, sex, BMI, SBP, DBP, WC, smoking and drinking habits, hypertension, diabetes, FPG, hsCRP and HOMA-IR. Multivariate analysis showed that, among plaque subtypes, only NCAP showed a significant association with apoB/A1 ratio. When we calculated the ORs for significant coronary stenosis in men and women separately, we observed significance only in men (Table 3).

ROC analysis revealed that the optimal apoB/A1 ratio cutoff for detecting significant coronary stenosis by MDCT was 0.58, which had a sensitivity of 70.2% and a specificity of 48.2% (Fig. 2, AUC, 0.61; 95% CI, 0.58-0.63, P < 0.001). Using this cut-off value, the OR for significant coronary stenosis was 2.06 (95% CI, 1.31-3.24, P = 0.002) after adjusting for age, sex, BMI, SBP, DBP, WC, smoking and drinking habits, hypertension, diabetes, FPG, hsCRP and HOMA-IR.