Effect on lipid, complete blood count and blood proteins of a

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ORIGINAL ARTICLE
Effect on lipid, complete blood count and blood
proteins of a standardized preparation for drawing
blood: a randomized controlled trial
Norm R.C. Campbell, MD
Alun L. Edwards, MB BChir
Rollin Brant, PhD
Charlotte Jones, MD
Diana Mitchell, RN
Dr. Campbell is with the Division of General Internal
Medicine and Geriatrics, Division of Pharmacology and
Therapeutics, and the Cardiovascular Research Group;
Drs. Edwards and Jones and Ms. Mitchell are with the
Division of Endocrinology and Metabolism; Dr. Brant is
with the Community Health Sciences, University of Calgary, Calgary, Alta.
Medical subject headings: blood cell count; blood specimen
collection; cholesterol; hyperlipidemia; laboratories; laboratory
techniques and procedures; lipids; lipoproteins; phlebotomy;
randomized controlled trials; research; tourniquets
(Original manuscript submitted May 9, 2000; received in
revised form July 21, 2000 accepted July 27, 2000.)
Clin Invest Med 2000;23(6):350-4.
© 2000 Canadian Medical Association
Abstract
Objective: To compare a standardized recommended
procedure for drawing blood to measure blood lipid and
lipoprotein levels with the procedure commonly used in
clinical practice. The aim was to see if hemoconcentration and spuriously elevated lipid levels could be
avoided.
Design: An open randomized crossover clinical trial.
Setting: The University of Calgary.
Patients: Twenty-five patients with dyslipidemia.
Interventions: Blood drawing using a standardized
procedure in which the patient remained seated for 5
minutes before blood collection and tourniquet use was
minimized or avoided.
Main outcome measures: Differences in lipid levels
between the usual clinical procedure and the recommended procedure for drawing blood.
Results: Prior to drawing blood, laboratories have sat
patients for an average of 1.4 minutes (95% CI, 0.9 to
1.9) and used a tourniquet in every patient. In the standardized procedure, patients rested for an average of 5.6
minutes (95% CI 5.0 to 6.2), and a tourniquet was used
briefly in only 3 of 23 patients. There were no differ350
ences in lipid and lipoprotein values and no clinically
significant difference in hemoglobin or albumin levels
or in the calculation of hemoconcentration.
Conclusions: Efforts to rest patients and avoid tourniquet use when drawing blood for assessment of lipid
levels are unlikely to be useful.
Résumé
Objectif : Comparer une procédure recommandée normalisée pour prélever du sang afin de mesurer les taux
sanguins de lipides et de lipoprotéines à la procédure
utilisée couramment en pratique clinique. On voulait
déterminer s’il était possible d’éviter l’hémoconcentration et les taux de lipides élevés transitoires.
Conception : Étude clinique croisée randomisée ouverte.
Contexte : Université de Calgary.
Patients : Vingt-cinq patients atteints de dyslipidémie.
Interventions : Prélèvement de sang au moyen d’une
procédure normalisée au cours de laquelle le patient est demeuré assis 5 minutes avant le prélèvement du sang et l’on
a réduit au minimum ou évité l’utilisation du tourniquet.
Principales mesures de résultats : Différences des taux
Clin Invest Med • Vol 23, no 6, décembre 2000
Standard versus standardized blood drawing for lipid assessment
de lipides établis à la suite de la procédure clinique
habituelle et de la procédure recommandée de prélèvement du sang.
Résultats : Avant de prélever le sang, les laboratoires
font asseoir les patients en moyenne 1,4 minute (IC 95
%, 0,9 à 1,9) et utilisent un tourniquet pour chaque patient. Selon la procédure normalisée, le patient prend une
pause de 5,6 minutes en moyenne (IC 95 %, 5,0 à 6,2) et
l’on a utilisé brièvement le tourniquet chez 3 patients
seulement sur 23. On n’a constaté aucune différence entre les taux de lipides et de lipoprotéines ni aucune différence significative sur le plan clinique entre les taux
d’hémoglobine et d’albumine ou entre les calculs de
l’hémoconcentration.
Conclusions : Il y a peu de chances que les efforts
visant à faire reposer les patients et à éviter d’utiliser le
tourniquet lorsqu’on prélève du sang pour la détermination des taux de lipides portent fruit.
Introduction
was used. It was not possible for investigators or patients to be blinded to the allocated procedures. The
laboratory technicians determining lipid values were
blinded to the allocated procedure, and those drawing blood routinely in the clinical laboratories were
unaware of the study design. The nurse performing
the standardized procedure was aware of the study
design and its objectives. The random sequence for
blood drawing was computer generated with the
allocations enclosed in sealed envelopes.
Dyslipidemias are strong reversible risk factors for cardiovascular disease. Various guidelines advocate periodic screening of lipid levels for healthy adults and for
those with established cardiovascular disease.1–5 Risk
classification and therapeutic monitoring require accurate assessment of lipid and lipoprotein levels. Unfortunately lipid levels in clinical assessments vary substantially.6–10 It is estimated that in up to 40% of
patients the risk classification changes on retesting.11
Although some of the variability associated with
assessing lipids is related to laboratory procedures,
the large proportion is related to biological variation
(i.e., before the blood sample reaches the laboratory).6,12,13 The method of drawing blood has been reported to affect the lipid value. In particular, the use
of a tourniquet and the time that the patient remains
seated before blood is drawn are reported to be important factors causing lipid levels to be artificially
elevated.14–17 Interestingly, only one of the recent major trials19 examining the benefits of lipid-lowering
agents indicated compliance with recommendations
for standardized blood drawing.18–23
We compared the effect of usual clinic phlebotomy
to a standardized, recommended procedure (resting patients 5 minutes before blood sampling and avoiding
tourniquet use) on measured blood lipid levels. Further, we examined the compliance with guidelines not
to use a tourniquet and to seat a patient for 5 minutes
before drawing blood for serum lipid measurement in
the Calgary Regional Health Authority laboratories.
Methods
Design
An open, randomized, controlled, crossover design
Clin Invest Med • Vol 23, no 6, December 2000
Patients
Patients were selected from those referred to a lipid
clinic for either nurse-dietician counselling or for
subspecialty lipid consultation by an endocrinologist.
Inclusion criteria included patients with dyslipidemia
on no treatment or on stable lifestyle modification or
pharmacotherapy for more than 3 months. Exclusion
criteria were unstable medical conditions or medical
conditions that in the opinion of the investigators
would influence the results or impede the ability of
the patient to complete the study (diabetes mellitus
on pharmacotherapy, elevated serum creatinine concentration, diuretic therapy, diseases of the pulmonary, cardiovascular, gastrointestinal or central
nervous system that interfere with the ability to perform activities of daily living). Patients with stable
medical conditions and medical therapy were not
excluded. Twenty-five patients (15 women, 10 men),
participated in the study The mean and (standard deviation) age of the group was 58.8 (8.7) years, and
the body mass index was 30.1 (5). Two patients
dropped out: in one venous access was lacking, and
the other would not return for the second visit. In addition to dyslipidemia, associated conditions included
ischemic heart disease (3 patients), depression (3),
and hypothyroidism, prostate hypertrophy, aortic
351
Campbell et al
stenosis, asthma, migraine, hepatitis B, fibromyalgia
and unspecified arthritis (1 each). Medications being
taken included pravastatin (3 patients), amlodipine
(3), flouxetine (3), and micronized fenofibrate, simvastatin, salbutamol, thyroxine, finasteride, metoprolol, acetylsalicylic acid, omeprazole (1 each).
Blood collection
The fasting procedure for all patients before blood
drawing was that of the National Cholesterol
Education Program.3 A research nurse performed the
experimental standardized blood collection: the
patients were all seated for 5 minutes before blood
collection and whenever possible tourniquet use was
avoided or minimized. For the usual blood drawing,
patients were able to select any laboratory in the
Calgary Regional Health Authority. Patients were requested to record the duration of time in the seated
position before blood drawing and whether or not a
tourniquet was used. The blood drawings were 1 to 4
weeks apart. Patients were advised not to change
diet or activity during the study and their medications were not changed. The patients had blood
drawn at 12 different sites.
Handling of blood samples
Blood samples were drawn into 8-mL SST (clot)
tubes. The samples were centrifuged and frozen at
–18 °C for up to 28 weeks. Samples from the same
patients were measured at the same time to avoid
interassay variation.
Statistical analysis
The order of the blood collection method was randomized according to a crossover design. The resultant data were first inspected descriptively for distributional properties. Subsequently the effects of 2
methods of measurement were compared by the
method of Hills–Armitage.23 Statistical computation
was performed with the use of S-Plus.24
Results
The mean length of time that patients remained
352
seated before blood collection was 1.4 minutes (95%
CI, 0.9 to 1.9) in the usual clinical procedure compared with 5.6 minutes (95% CI 5.0 to 6.2) in the
standardized procedure. Tourniquets were used in all
of the patients for the usual clinical procedure but
were required in only 3 patients for the standardized
collections.
There were no significant differences in serum
lipid levels between the standardized and the usual
clinical method (Table 1). In particular, the trial
ruled out an increase of greater than 0.2 mmol/L total cholesterol, 0.2 mmol/L high-density lipoprotein
cholesterol, 0 mmol/L low-density lipoprotein cholesterol and 0.4 mmol/L triglycerides. There were
minor changes consistent with hemoconcentration of
albumin, hemoglobin and hematocrit when the clinical laboratory drew blood. This was reflected by a
decrease in vascular volume as calculated by the
formula of Strauss and colleagues,25 which is based
on hemoglobin and hematocrit.
Comment
The usual clinical method of blood collection does
not follow standardized recommendations. However,
our study had the power to rule out (with at least
95% certainty) increases in lipid or lipoprotein concentrations greater than 0.2 mmol/L (triglycerides
greater than 0.4 mmol/L), associated with the lack of
standardized technique. The changes in lipids and
Table 1: Percentage changes (and 95% CI) in blood and serum
levels from a standardized procedure when blood was drawn by a
routine laboratory procedure
After
% change from
standardized
standardized
95% CI of
Measurement
procedure
procedure
% change
Hemoglobin, g/L
143
+3.6
1.2, 6.0
Calculated vascular
100
–2.0
–3.2, –0.8
volume, %
Total protein, g/L
69
+1.1
–0.1, 2.2
Albumin, g/L
40
+0.9
0.1, 1.7
Total cholesterol,
5.7
–0.1
–0.3, 0.2
mmol/L
HDL cholesterol,
1.2
+0.1
–0.05, 0.2
mmol/L
LDL cholesterol,
3.6
–0.2
–0.4, 0.0
mmol/L
Triglycerides,
2.2
0
–0.4, 0.4
mmol/L
HDL = high-density lipoprotein, LDL = low-density lipoprotein.
Clin Invest Med • Vol 23, no 6, décembre 2000
Standard versus standardized blood drawing for lipid assessment
lipoproteins of this magnitude are unlikely to affect
clinical decisions and therefore efforts to standardize
the method of blood drawing in lipid assessments
are not necessary.
There are several potential reasons why we did not
find an effect of tourniquet use or rest in the seated
position whereas other, well-conducted studies
found substantial effects. Ours was an effectiveness
study that examined usual clinical practice whereas
other studies examined the extremes of clinical practice (efficacy studies). Patients in usual clinical practice tend to wait in the seated position in the laboratory until called for blood drawing. Research
conducted on the necessity of sitting for 5 to 10 minutes had patients standing before blood collection as
a comparison.16,17 Additionally, the duration of use
and pressure exerted by the tourniquet before blood
sampling may be factors determining the degree of
hemoconcentration. In this study a tourniquet was
used by all clinical laboratories during blood sampling but in only 3 of the experimental blood collections. The duration and pressure exerted by tourniquet use are likely less in the clinical laboratories
than in research studies that demonstrated hemoconcentration due to tourniquet use.14,15 Tourniquet use
and lack of standardized rest have similar effects on
blood lipid levels. The absence of any discernible
lipid effect in clinical practice, where both the lack
of a standardized rest and the use of a tourniquet
were common, suggests that expending resources to
deter tourniquet use or to standardize time in the
seated position will not be beneficial. However, there
may be unusual circumstances (e.g., patients with
difficult venous access) in which patient position and
tourniquet use may cause hemoconcentration. Technicians drawing blood will need to use judgement,
and clinicians and laboratory scientists need to be
aware of the potential for spurious results.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
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Reprint requests to: Dr. Norm Campbell, Department of Medicine, Faculty of Medicine, University
of Calgary, 3330 Hospital Dr. NW, Calgary, AB
T2N 4N1; fax 403 283-6151, ncampbel@
ucalgary.ca
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