Online Appendix for the following JACC article
TITLE: Friedewald Estimated Versus Directly Measured Low-Density Lipoprotein Cholesterol
and Treatment Implications
AUTHORS: Seth S. Martin, MD, Michael J. Blaha, MD, Mohamed B. Elshazly, MD, Eliot A.
Brinton, MD, Peter P. Toth, MD, PHD, John W. McEvoy, MB BCh, Parag H. Joshi, MD,
Krishnaji R. Kulkarni, PHD, Patrick D. Mize, PHD, Peter O. Kwiterovich, MD, Andrew P.
DeFilippis, MD, Roger S. Blumenthal, MD, Steven R. Jones, MD
Extended Methods on Lipid Measurements
Accuracy for total cholesterol and HDL-C
VAP accuracy for total cholesterol and HDL-C was examined by yearly (2007 to 2012)
random split sample comparison (n = 330) with results from Washington University’s Core
Laboratory for Clinical Studies (St. Louis, MO) (r=0.986, r=0.969, respectively; bias 0.6%, 1.7%,
respectively).
Precision for VAP and Triglycerides
VAP precision was examined in 32 samples, which were each analyzed 17 times with
different sets of instruments. The intra-assay coefficient of variation was 2.9% for direct LDL-C,
2.6% for total cholesterol, and 2.9% for HDL-C. Triglyceride measurements had a coefficient of
variation of 0.93%.
LDL-C Definitions
The purpose of this section is to provide additional information to clarify definitions of LDL-C
(conventional Friedewald definition versus biochemical definition).
First consider the equation for total cholesterol:
Total cholesterol = HDL-C + LDL-C + IDL-C + Lp(a)-C + VLDL-C + Chylomicron-C
Because chylomicrons generally contribute an insignificant portion to total blood cholesterol, the
equation is conventionally simplified to:
Total cholesterol = HDL-C + LDL-C + IDL-C + Lp(a)-C + VLDL-C
Therefore:
LDL-C = Total cholesterol – HDL-C – IDL-C – Lp(a)-C – VLDL-C
Compare this equation with the Friedewald equation:
LDL-C = Total cholesterol – HDL-C – VLDL-C estimated as Triglycerides/5
We see that IDL-C and Lp(a)-C are folded into LDL-C. Therefore, Friedewald LDL-C, by
definition, includes actual LDL-C (commonly referred to as “real LDL-C”) plus these non-LDLC fractions. That is, the LDL-C of common parlance is the sum of the 3 subfractions included in
the original Friedewald estimate, distinct from LDL-C as it is defined biochemically.
Some direct LDL-C assays measure real LDL-C only, such as the assay used in the Heart
Protection Study. To keep with the conventional Friedewald definition, the direct LDL-C
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measurement in our study is the sum of real LDL-C + IDL-C + Lp(a)-C; an equivalent definition
is total cholesterol – HDL-C – direct VLDL-C, which yields essentially identical results in our
sample.
Appendix Table 1: Additional Clinical Laboratory Data in a Random Subsample
Full Subsample
Accurately Classified
Misclassified
N=3,107
N=2,647
N=460
Hemoglobin A1c − %
5.7 (5.5-6.1)
5.7 (5.4-6.1)
5.8 (5.5-6.4)
Glucose − mg/dL
95 (88-107)
95 (87-105)
99 (90-115)
Insulin − µU/mL
9.4 (5.9-16.1)
9.0 (5.7-15.1)
13.2 (8.0-24.6)
Homeostasis Model Assessment of Insulin
2.2 (1.4-4.3)
2.1 (1.3-3.9)
3.3 (1.8-7.2)
1.9 (0.8-4.4)
1.8 (0.8-4.2)
2.5 (1.2-5.2)
11.0 (8.9-14.8)
11.0 (8.9-14.7)
11.3 (9.0-15.4)
Creatinine − mg/dL
0.9 (0.8-1.0)
0.9 (0.8-1.0)
0.9 (0.8-1.0)
Cystatin C − mg/L
1.1 (0.9-1.3)
1.1 (0.9-1.3)
1.1 (0.9-1.3)
Blood urea nitrogen − mg/dL
16.3 (13.1-20.6)
16.3 (13.2-20.6)
16.6 (13.0-20.6)
*Estimated GFR, ml/min/1.73 m2
67.4 (49.3-84.1)
67.8 (49.2-84.2)
64.2 (50.6-83.1)
1,239 (39.9)
1,051 (39.7)
188 (40.9)
3.9 (3.7-4.1)
3.9 (3.7-4.1)
3.9 (3.7-4.1)
21 (18-26)
21 (18-26)
22 (18-27)
Resistance
hs-C-reactive protein – mg/L
Homocysteine – µmol/L
<60 ml/min/1.73 m2 – no. (%)
Albumin – g/dL
Aspartate aminotransferase – U/L
2
Alanine aminotransferase – U/L
22 (16-30)
22 (16-29)
25 (19-36)
Creatine phosphokinase – U/L
96 (65-143)
96 (64-143)
96 (64-143)
N-Terminal pro-B-type natriuretic peptide, pg/ml
74 (25-175)
74 (35-171)
77 (35-200)
Thyroid stimulating hormone – uIU/ml
1.5 (1.0-2.2)
1.5 (1.0-2.2)
1.5 (1.0-2.2)
Values are expressed as median (25th-75th percentile).
*Calculated by the CKD-EPI cystatin C equation as 133x(cystatin C/0.8) -0.499x0.996Age[x0.932 if female] if serum
cystatin C ≤0.8 mg/liter and 133x(cystatin C/0.8) -1.328x0.996Age[x0.932 if female] if serum cystatin C >0.8 mg/liter.
Inker LA, Schmid CH, Tighiouart H, Eckfeldt JH, Feldman HI, Greene T, Kusek JW, Manzi J, Van Lente F, Zhang
YL, Coresh J, Levey AS; CKD-EPI Investigators. Estimating glomerular filtration rate from serum creatinine and
cystatin C. N Engl J Med 2012;367:20-9.
Appendix Figure 1: Impact of Triglyceride Level on Absolute Difference in Friedewald and
Direct LDL-C. The absolute difference (Friedewald – direct LDL-C) is shown across four
triglyceride strata in the full study sample of 1,310,440 adults. The density of data is expressed
by different shades of color, which represent increasing densities of patients per pixel, from light
blue to purple. Color pixels below the dark horizontal lines represent cases in which Friedewald
LDL-C is lower than direct LDL-C and vice versa for color pixels above the line.
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Appendix Figure 2: Impact of LDL-C Level on Absolute Difference in Directly Measured
and Estimated VLDL-C. The absolute difference in directly measured VLDL-C minus
triglycerides/5 is shown across Friedewald LDL-C levels in the full study sample of 1,310,440
adults. The density of data is expressed by different shades of color, which represent increasing
densities of patients per pixel, from light blue to purple. Color pixels below the dark horizontal
line represent cases in which direct VLDL-C is lower than triglycerides/5 (and in turn,
Friedewald LDL-C is generally lower than direct LDL-C) and vice versa for color pixels above
the line. The similarity of this density pattern with Figure 2A, where the y axis is Friedewald
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minus direct LDL-C, illustrates that the difference in Friedewald and direct LDL-C is compatible
with VLDL-C estimation as opposed to error in total cholesterol and HDL-C measurements.
Pediatric Results
Replicating key findings from our adult sample, the table and figures below analyze
10,151 pediatric patients (age <18 years) with triglycerides <400 mg/dl wherein
pharmacotherapy, including statins, is less likely. Our methods were the same as in adults,
including data collection, lipid measurement, and statistical analysis.
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Pediatric Table: Reclassification by Direct LDL-C in Strata of Friedewald LDL-C and
Triglycerides
Concordant
Reclassified
N=8,726
N=1,425
no./total no. in group (%)
Friedewald LDL-C
≥190 mg/dl
205/219 (93.6)
14/219 (6.4)
160 to 189 mg/dl
258/292 (88.4)
34/292 (11.6)
130 to 159 mg/dl
788/900 (87.6)
112/900 (12.4)
100 to 129 mg/dl
2,360/2,709 (87.3)
349/2,709 (12.9)
70 to 99 mg/dl
3,645/4,203 (86.7)
558/4,203 (13.3)
70 mg/dl
1,470/1,828 (80.4)
358/1,828 (19.6)
<100 mg/dl
5,075/5,494 (92.4)
419/5,494 (7.6)
100 to 149 mg/dl
2,325/2,597 (89.5)
272/2,597 (10.5)
150 to 199 mg/dl
856/1,130 (75.8)
274/1,130 (24.3)
470/930 (50.5)
460/930 (49.5)
Triglycerides
200 to 399 mg/dl
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Pediatric Figure 1: Impact of LDL-C Level on Absolute Difference in Friedewald and
Direct LDL-C. The absolute difference (Friedewald – direct LDL-C) is shown across
Friedewald LDL-C levels in the pediatric sample (n = 10,151). The density of data is expressed
by different shades of color, which represent increasing densities of patients per pixel, from light
blue to deep purple. Color pixels below the dark horizontal line represent cases in which
Friedewald LDL-C is lower than direct LDL-C and vice versa for color pixels above the line.
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Pediatric Figure 2: Impact of Triglyceride Level on Absolute Difference in Friedewald and
Direct LDL-C. By triglyceride strata, the absolute difference (Friedewald – direct LDL-C) is
shown across Friedewald LDL-C levels in the pediatric sample (n = 10,151). The density of data
is expressed by different shades of color, which represent increasing densities of patients per
pixel, from light blue to deep purple. Color pixels below the dark horizontal line represent cases
in which Friedewald LDL-C is lower than direct LDL-C and vice versa for color pixels above the
line.
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Pediatric Figure 3: Absolute Difference in Friedewald and Direct LDL-C by Triglyceride
Strata at LDL-C Levels in the Treatment Range for High-Risk Patients and Treatment
Group Reclassification. (A) Absolute difference (Friedewald – direct LDL-C) in Friedewald
LDL-C groups of 70 to 99 mg/dl and <70 mg/dl with sub-stratification into four triglyceride
groups (each assigned a color, as depicted). Proportion reclassified by direct LDL-C if (B)
Friedewald LDL-C 70 to 99 mg/dl or (C) Friedewald LDL-C <70 mg/dl.
A
B
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C
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