Supplemental material - Report on the comparison of laboratory measurements
Bias in protein and potassium intake collected with 24-h recalls (EPIC-soft) is rather
comparable across European populations. European Journal of Nutrition.
Sandra P. Crispima,b, Anouk Geelenb, Jeanne H. M. de Vriesb, Heinz Freislingc, Olga Souvereinb, Paul J. M.
Hulshofb, Marga C. Ockec, Hendriek Boshuizenb,d, Lene F. Andersend, Jiri Rupriche, Willem De Keizerf,g,
Inge Huybrechtsc,h, Lionel Lafayh, Maria Santucci de Magistrisi, Fulvio Riccerij, Rosario Tuminok, Vittorio
Kroghl, H. Bas Bueno-de-Mesquitad,m, Joline WJ Beulensn, Marie-Christine Boutron-Ruaulto, Androniki
Naskap, Francesca L Croweq, Heiner Boeingr, Alison McTaggarts, Rudolf Kaakst, Pieter van’t Veerb, Nadia
Slimanic
a
Division of Human Nutrition, Wageningen University, Wageningen, The Netherlands.
International Agency for Research on Cancer (IARC), Lyon, France.
National Institute for Public Health and the Environment (RIVM), Bilthoven, The Netherlands.
d
Department of Nutrition, Institute of Basic Medical Sciences, University of Oslo, Oslo, Norway.
e
National Institute of Public Health, Department for Food Safety and Nutrition, Brno, Czech Republic.
f
Department of Nutrition and Dietetics, University College Ghent, Ghent, Belgium.
g
Department Public Health, Ghent University, Ghent, Belgium.
h
French Agency for Food, Environmental and Occupational Health Safety (ANSES), Paris, France.
i
Department of Public Clinical and Experimental Medicine, Federico II, Naples, Italy.
j
Human Genetics Foundation (HUGEF), Turin, Italy.
k
Cancer Registry and Histopathology Unit, "Civile M.P.Arezzo" Hospital, Ragusa, Italy.
l
Nutritional Epidemiology Unit, Fondazione IRCCS Istituto Nazionale Tumori, Milan, Italy.
m
Department of Gastroenterology and Hepatology, University Medical Centre Utrecht (UMCU), Utrecht, The Netherlands.
n
Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, Utrecht, The Netherlands.
o
Inserm, Centre for Research in Epidemiology and Population Health, Villejuif, France.
p
WHO Collaborating Center for Food and Nutrition Policies, Department of Hygiene, Epidemiology and Medical Statistics, University of Athens Medical School,
Athens, Greece.
q
Cancer Epidemiology Unit, Nuffield Department of Clinical Medicine University of Oxford, Oxford, United Kingdom.
r
Department of Epidemiology, German Institute of Human Nutrition, Potsdam-Rehbrücke, Germany
s
Department of Public Health and Primary Care, University of Cambridge, Cambridge, United Kingdom.
t
Division of Cancer Epidemiology, German Cancer Research Center, Heidelberg, Germany.
b
c
With the purpose of harmonizing biomarker data used in the paper ‘Bias in protein and potassium
intake collected with 24-h recalls (EPIC-Soft) is rather comparable across European populations’,
a calibration study was conducted among laboratories that performed chemical analyses of urine
samples from the EPIC and EFCOVAL studies.
Nutrients and specimens in the calibration study: The amount of urinary nitrogen and urinary
potassium of 45 samples from the EPIC cohort were determined at both the MRC Dunn Clinical
Nutrition Centre in Cambridge (EPIC) in the early 2000’s and at the Division of Human Nutrition at
Wageningen University (EFCOVAL) in 2008.
Methodology: Agreement between the two biomarker measurements determined by the
Cambridge and Wageningen laboratories was assessed using Bland-Altman plots and Pearson’s
coefficients of correlation. The paired t-test was used to test for significant differences (p<0.05)
between the mean biomarker measurements from the two laboratories. In addition, comparability
of laboratory methods used in EPIC and EFCOVAL labs was further substantiated by evaluating
standard reference materials and quality control procedures (e.g., inter-laboratory proficiency
tests) of the lab measurements.
Linear regression analyses were carried out to generate calibration equations between the
biomarker measurements from the two laboratories. Protein and potassium values which were +/3SD from the mean were considered outliers (n = 1 for nitrogen and for potassium) and excluded
from the regression analyses.
Results: A good agreement and correlation was seen between the measurements from the two
laboratories based on the Bland-Altman plots (Figure S1 and S2) and Pearson’s correlation
coefficients (r>0.97), respectively (Table S1). No significant differences were seen between the
mean nitrogen (p=0.10) and potassium (p=0.68) measurements from the two laboratories.
Based on those results, we judged that calibration of data between EFCOVAL and EPIC studies
was not necessary and original biomarker data of the two studies was used in the main analyses
of the paper.
Because the calibration study was limited by the rather small sample size, we still estimated the
calibration equations and performed a sensitivity analysis using the calibrated data in the
multilevel analysis. The EFCOVAL data was calibrated using the following regression formulas:
EFCOVAL_N_calibrated = 0.92 x EFCOVAL_N + 0.03
EFCOVAL_K_calibrated = 0.92 x EFCOVAL_K + 1.87
Standard reference materials and inter-laboratory proficiency tests of the laboratory procedures in
the two laboratories indicated no discrepancies between the analyses performed. Thus, there
was no evidence to opt for having one or the other laboratory as reference in the calibration
procedure, and the Cambridge lab (from the EPIC study) was chosen.
Table S1 – Comparison of means (SD) and Pearson’s correlation coefficients between biomarker
measurements determined by the Wageningen and Cambridge laboratories in the calibration
study
Biomarker
Urinary nitrogen (g/100ml)
Urinary potassium (mmol/L)
a
b
Paired t-test
Pearson correlation
n
45
45
Wageningen
0.70 (0.3)
42.1 (17.3)
Cambridge
0.68 (0.3)
40.6 (16.2)
pa
0.10
0.68
rb
0.97
0.99
Figure S1 - Bland-Altman plot for nitrogen measurements from the Cambridge and Wageningen
laboratories.
Figure S2 - Bland-Altman plot for potassium measurements from the Cambridge and
Wageningen laboratories.