title: Testing bias in clinical databases: methodological considerations.
abstract: BACKGROUND
  Laboratory testing in clinical practice is never a random process. In this study we evaluated testing bias for neutrophil counts in clinical practice by using results from requested and non-requested hematological blood tests.
  
  
  METHODS
  This study was conducted using data from the Utrecht Patient Oriented Database. This clinical database is unique, as it contains physician requested data, but also data that are not requested by the physician, but measured as result of requesting other hematological parameters. We identified adult patients, hospitalized in 2005 with at least two blood tests during admission, where requests for general blood profiles and specifically for neutrophil counts were contrasted in scenario analyses. Possible effect modifiers were diagnosis and glucocorticoid use.
  
  
  RESULTS
  A total of 567 patients with requested neutrophil counts and 1,439 patients with non-requested neutrophil counts were analyzed. The absolute neutrophil count at admission differed with a mean of 7.4 x 109/l for requested counts and 8.3 x 109/l for non-requested counts (p-value < 0.001). This difference could be explained for 83.2% by the occurrence of cardiovascular disease as underlying disease and for 4.5% by glucocorticoid use.
  
  
  CONCLUSIONS
  Requests for neutrophil counts in clinical databases are associated with underlying disease and with cardiovascular disease in particular. The results from our study show the importance of evaluating testing bias in epidemiological studies obtaining data from clinical databases.
authors.affiliation: Faculty of Science, Division of Pharmacoepidemiology and Pharmacotherapy, Utrecht Institute of Pharmaceutical Sciences, Utrecht University, The Netherlands. H.G.M.Leufkens@uu.nl.
authors.forename: Karin J
authors.initials: KJ
authors.lastname: Velthove
authors.forename: Hubert Gm
authors.initials: HG
authors.lastname: Leufkens
authors.forename: Patrick C
authors.initials: PC
authors.lastname: Souverein
authors.forename: René C
authors.initials: RC
authors.lastname: Schweizer
authors.forename: Wouter W
authors.initials: WW
authors.lastname: van Solinge
references.citation: Pharmacoepidemiol Drug Saf. 2004 Jul;13(7):437-41
references.pmid: 15269927
references.citation: Clin Chem Lab Med. 2007;45(1):13-9
references.pmid: 17243908
references.citation: Arch Intern Med. 1999 Jan 11;159(1):65-70
references.pmid: 9892332
references.citation: Lancet. 2006 Aug 26;368(9537):804-13
references.pmid: 16935691
references.citation: Exp Gerontol. 2004 Apr;39(4):615-9
references.pmid: 15050297
references.citation: J Clin Epidemiol. 2003 Jun;56(6):501-6
references.pmid: 12873643
references.citation: Eur J Epidemiol. 1999 Oct;15(9):773-81
references.pmid: 10608355
references.citation: Br Med J. 1968 Apr 27;2(5599):199-205
references.pmid: 5653025
references.citation: Clin Chem Lab Med. 2009;47(5):573-8
references.pmid: 19397484
references.citation: J Vasc Surg. 2005 Apr;41(4):610-7
references.pmid: 15874924
references.citation: Emerg Themes Epidemiol. 2009 Feb 27;6:2
references.pmid: 19250518
references.citation: Eur J Epidemiol. 2007;22(4):267-72
references.pmid: 17342451
references.citation: Circulation. 1989 Mar;79(3):549-56
references.pmid: 2537159
references.citation: Eur Respir J. 1998 Oct;12(4):820-4
references.pmid: 9817152
references.citation: J Clin Epidemiol. 2005 Apr;58(4):323-37
references.pmid: 15862718
references.citation: Med Care. 2007 Oct;45(10 Supl 2):S13-5
references.pmid: 17909371
references.citation: Emerg Themes Epidemiol. 2005 Sep 01;2:8
references.pmid: 16137327
references.citation: Am J Respir Crit Care Med. 1999 Nov;160(5 Pt 1):1532-9
references.pmid: 10556116
references.citation: J Chronic Dis. 1979;32(1-2):51-63
references.pmid: 447779
references.citation: Eur J Epidemiol. 2006;21(3):167-70
references.pmid: 16547830
references.citation: Pharmacoepidemiol Drug Saf. 2008 Jun;17(6):535-45
references.pmid: 18265414
references.citation: J Public Health Med. 1999 Sep;21(3):299-304
references.pmid: 10528957
references.citation: Eur Respir J. 1999 May;13(5):1198-208
references.pmid: 10414427