Homocysteine testing has emerged from an obscure esoteric laboratory test,
to one of the most important screening examinations for determining risk in
atherosclerotic disease.
OUTLINE
| ANALYTICAL METHOD |
For many years, HPLC was the standard, now, enzyme immunoassays
are becoming increasingly popular
These EIA techniques are based upon similar techniques |
| BIOCHEMICAL THEORY |
Homocysteine oxidizes low density lipoprotein which
may precipitate atherosclerosis and is toxic to the endothelium
Decreases expression of thombomodulin on the surface of endothelial
cells-thrombomodulin interacts with protein C activation leading to
inhibition of anticoagulant pathway, leading to thrombosis
Activates platelets and increases platelet aggregation |
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Ann Intern Med 1999;131:363-375
Enhances smooth muscle cell proliferation, intimal-medial wall thickness,
endothelial cell injury, thromboxane A2 formation, lipid abnormalities,
LDL oxidation, and lipoprotein (a) binding to fibrin |
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The reduced form is the most active form because of the
sulfhydryl group |
| MTHFR 677C-T POLYMORPHISM |
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MTHFR 677C-->T Polymorphism and Risk of Coronary Heart Disease: A
Meta-analysis.
Klerk M, Verhoef P, Clarke R, Blom HJ, Kok FJ, Schouten EG.
Wageningen Centre for Food Sciences and Division of Human Nutrition
and Epidemiology, Wageningen University, PO Box 8129, 6700 EV Wageningen,
the Netherlands. |
JAMA 2002 Oct 23;288(16):2023-31 Abstract quote CONTEXT: In observational
studies, individuals with elevated levels of plasma homocysteine tend
to have moderately increased risk of coronary heart disease (CHD). The
MTHFR 677C-->T polymorphism is a genetic alteration in an enzyme
involved in folate metabolism that causes elevated homocysteine concentrations,
but its relevance to risk of CHD is uncertain.
OBJECTIVE: To assess the relation of MTHFR 677C-->T polymorphism
and risk of CHD by conducting a meta-analysis of individual participant
data from all case-control observational studies with data on this polymorphism
and risk of CHD. DATA SOURCES: Studies were identified by searches of
the electronic literature (MEDLINE and Current Contents) for relevant
reports published before June 2001 (using the search terms MTHFR and
coronary heart disease), hand searches of reference lists of original
studies and review articles (including meta-analyses) on this topic,
and contact with investigators in the field.
STUDY SELECTION: Studies were included if they had data on the MTHFR
677C-->T genotype and a case-control design (retrospective or nested
case-control) and involved CHD as an end point. Data were obtained from
40 (34 published and 6 unpublished) observational studies involving
a total of 11 162 cases and 12 758 controls.
DATA EXTRACTION: Data were collected on MTHFR 677C-->T genotype,
case-control status, and plasma levels of homocysteine, folate, and
other cardiovascular risk factors. Data were checked for consistency
with the published article or with information provided by the investigators
and converted into a standard format for incorporation into a central
database. Combined odds ratios (ORs) for the association between the
MTHFR 677C-->T polymorphism and CHD were assessed by logistic regression.
DATA SYNTHESIS: Individuals with the MTHFR 677 TT genotype had a 16%
(OR, 1.16; 95% confidence interval [CI], 1.05-1.28) higher odds of CHD
compared with individuals with the CC genotype. There was significant
heterogeneity between the results obtained in European populations (OR,
1.14; 95% CI, 1.01-1.28) compared with North American populations (OR,
0.87; 95% CI, 0.73-1.05), which might largely be explained by interaction
between the MTHFR 677C-->T polymorphism and folate status.
CONCLUSIONS: Individuals with the MTHFR 677 TT genotype had a significantly
higher risk of CHD, particularly in the setting of low folate status.
These results support the hypothesis that impaired folate metabolism,
resulting in high homocysteine levels, is causally related to increased
risk of CHD.
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| IDEAL TESTING STATE |
Homocysteine in whole blood is extremely unstable and
if left at room temperature, the level may increase by 10% or more within
one hour
If left for 24 hours, levels may rise 35-75%
Ideally should be centrifuged within 15 minutes of collection and stored
on ice or stored on ice immediatly for later centrifugation |
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Fasting |
| REFERNCE RANGE |
Ann Intern Med 1999;131:331-339
Although most laboratories use 15 umol/L as the cutoff between normal
and abnormal, this recent study indicates that the reference range may
be much lower
This study excluded patients with know risk factors for hyperhomocysteinemia
including low vitamin B12 or folate intake, renal dysfunction, pregnant
or postmenopausal women |
| CLINICAL UTILITY |
CHARACTERIZATION |
| ISCHEMIC HEART DISEASE |
|
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Hyperhomocysteinemia measured by immunoassay: a valid measure of coronary artery atherosclerosis.
Stauffenberg MT, Lange RA, Hillis LD, Cigarroa J, Hsu RM, Devaraj S, Jialal I.
Department of Pathology, The University of Texas Southwestern Medical Center at Dallas, USA.
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| Arch Pathol Lab Med. 2004 Nov;128(11):1263-6. Abstract quote |
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CONTEXT: Homocysteine is emerging as a novel marker of atherothrombosis. Its role as an independent risk factor for cardiovascular disease is generally accepted. There is scanty data correlating homocysteine levels measured by immunoassay with cardiovascular disease. We previously validated a fluorescence polarization immunoassay for measuring homocysteine, which compared favorably with high performance liquid chromatography.
OBJECTIVE: To determine if homocysteine levels measured by immunoassay correlate with extent of atherosclerotic burden, as represented by degree of coronary artery stenosis determined by coronary angiography.
DESIGN: Fasting plasma samples were obtained from patients undergoing coronary angiography (N = 165). Homocysteine levels were measured by immunoassay and coronary artery stenosis was determined by coronary angiography.
RESULTS: Median coronary artery stenosis for the 3 homocysteine subgroups, less than 1.35, 1.35 to 6.75, and greater than 6.75 mg/L (<10, 10-15, and >15 micromol/L), was 75%, 90%, and 99%, respectively (P = .01 for trend). Also, folate and vitamin B12 levels decreased with increasing homocysteine levels (P = .01 and .04, respectively, for trend). Spearman's correlation showed a significant association between homocysteine level and coronary artery stenosis (r = 0.20; P = .009). When men and women were examined separately, the correlation was significant only for women (r = 0.30; P = .01).
CONCLUSION: Homocysteine levels, as measured by immunoassay, show a positive correlation with cardiovascular disease in women. Thus, this is a valid measure of atherosclerotic burden and, therefore, a reliable addition to the established laboratory repertoire for the assessment of cardiovascular disease.
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Lowering homocysteine in patients with ischemic stroke to prevent recurrent stroke, myocardial infarction, and death: the Vitamin Intervention for Stroke Prevention (VISP) randomized controlled trial.
Toole JF, Malinow MR, Chambless LE, Spence JD, Pettigrew LC, Howard VJ, Sides EG, Wang CH, Stampfer M.
Stroke Research Center, Department of Neurology, Wake Forest University School of Medicine, Winston-Salem, NC 27157, USA.
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| JAMA. 2004 Feb 4;291(5):565-75 Abstract quote. |
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CONTEXT: In observational studies, elevated plasma total homocysteine levels have been positively associated with ischemic stroke risk. However the utility of homocysteine-lowering therapy to reduce that risk has not been confirmed by randomized trials.
OBJECTIVE: To determine whether high doses of folic acid, pyridoxine (vitamin B6), and cobalamin (vitamin B12), given to lower total homocysteine levels, reduce the risk of recurrent stroke over a 2-year period compared with low doses of these vitamins. DESIGN: Double-blind randomized controlled trial (September 1996-May 2003).
SETTING AND PARTICIPANTS: 3680 adults with nondisabling cerebral infarction at 56 university-affiliated hospitals, community hospitals, private neurology practices, and Veterans Affairs medical centers across the United States, Canada, and Scotland.
INTERVENTIONS: All participants received best medical and surgical care plus a daily multivitamin containing the US Food and Drug Administration's reference daily intakes of other vitamins; patients were randomly assigned to receive once-daily doses of the high-dose formulation (n = 1827), containing 25 mg of pyridoxine, 0.4 mg of cobalamin, and 2.5 mg of folic acid; or the low-dose formulation (n = 1853), containing 200 microg of pyridoxine, 6 microg of cobalamin and 20 microg of folic acid.
MAIN OUTCOME MEASURES: Recurrent cerebral infarction (primary outcome); coronary heart disease (CHD) events and death (secondary outcomes).
RESULTS: Mean reduction of total homocysteine was 2 micromol/L greater in the high-dose group than in the low-dose group, but there was no treatment effect on any end point. The unadjusted risk ratio for any stroke, CHD event, or death was 1.0 (95% confidence interval [CI], 0.8-1.1), with chances of an event within 2 years of 18.0% in the high-dose group and 18.6% in the low-dose group. The risk of ischemic stroke within 2 years was 9.2% for the high-dose and 8.8% for the low-dose groups (risk ratio, 1.0; 95% CI, 0.8-1.3) (P =.80 by log-rank test of the primary hypothesis of difference in ischemic stroke between treatment groups). There was a persistent and graded association between baseline total homocysteine level and outcomes. A 3- micromol/L lower total homocysteine level was associated with a 10% lower risk of stroke (P =.05), a 26% lower risk of CHD events (P<.001), and a 16% lower risk of death (P =.001) in the low-dose group and a nonsignificantly lower risk in the high-dose group by 2% for stroke, 7% for CHD events, and 7% for death.
CONCLUSIONS: In this trial, moderate reduction of total homocysteine after nondisabling cerebral infarction had no effect on vascular outcomes during the 2 years of follow-up. However, the consistent findings of an association of total homocysteine with vascular risk suggests that further exploration of the hypothesis is warranted and longer trials in different populations with elevated total homocysteine may be necessary. |
| Mild to moderate elevation can increase risk of coronary
artery disease by 2.5 times |
JAMA 1995;274:1049-1057
Lowering levels by 5 umol/L may reduce the death rate from coronary artery
disease by 10% |
| Cardiac bypass patients with high baseline levels |
Circulation 2000;102:605-610
If these patients are not treated within 2.5 years, they have 2.6 x greater
risk of recurring cardiac events |
| Homocysteine concentrations in a German cohort of
500 individuals: reference ranges and determinants of plasma levels
in healthy children and their parents.
Rauh M, Verwied S, Knerr I, Dorr HG, Sonnichsen A, Koletzko B.
University Children's Hospital, Friedrich-Alexander-University,
Erlanen-Nuermberg, Erlangen, Federal Republic of Germany. |
Amino Acids 2001;20(4):409-18 Abstract quote
Elevated plasma homocysteine is a risk factor for cardiovascular disease
and a sensitive marker of inadequate vitamin B12 and folate status.
We studied 257 pupils (120 boys, 137 girls, aged 6-17 years) and their
parents (88 males, 172 females, aged 26-50 years). Our measurements
were part of a national Bavarian health and nutrition examination survey
evaluating cardiovascular risk factors. A mild hyperhomocysteinemia
(Hcys >15 micromol/l) occurred in 7% of the adults, but in none of the
children. Men had significantly higher Hcys levels than women (p<0.0001),
boys and girls had comparable concentrations. For adults and children,
Hcys correlated inversely with vitamin B12 and folate and positively
with the lean body mass and creatinine in serum, but not with cystatin
C. Genetic and nutritional factors are determinants of Hcys metabolism.
The correlation of Hcys and serum creatinine is dependent on the metabolic
link between Hcys production and creatine synthesis. |
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Effect of homocysteine-lowering therapy with folic acid, vitamin B(12),
and vitamin B(6) on clinical outcome after percutaneous coronary intervention:
the Swiss Heart study: a randomized controlled trial.
Schnyder G, Roffi M, Flammer Y, Pin R, Hess OM.
Division of Cardiology, Swiss Cardiovascular Center Bern, University
Hospital, Switzerland.
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JAMA 2002 Aug 28;288(8):973-9 Abstract quote
CONTEXT: Plasma homocysteine level has been recognized as an important
cardiovascular risk factor that predicts adverse cardiac events in patients
with established coronary atherosclerosis and influences restenosis
rate after percutaneous coronary intervention.
OBJECTIVE: To evaluate the effect of homocysteine-lowering therapy
on clinical outcome after percutaneous coronary intervention.
DESIGN, SETTING, AND PARTICIPANTS: Randomized, double-blind placebo-controlled
trial involving 553 patients referred to the University Hospital in
Bern, Switzerland, from May 1998 to April 1999 and enrolled after successful
angioplasty of at least 1 significant coronary stenosis (> or = 50%).
INTERVENTION: Participants were randomly assigned to receive a combination
of folic acid (1 mg/d), vitamin B(12) (cyanocobalamin, 400 micro g/d),
and vitamin B(6) (pyridoxine hydrochloride, 10 mg/d) (n = 272) or placebo
(n = 281) for 6 months.
MAIN OUTCOME MEASURE: Composite end point of major adverse events defined
as death, nonfatal myocardial infarction, and need for repeat revascularization,
evaluated at 6 months and 1 year. RESULTS: After a mean (SD) follow-up
of 11 (3) months, the composite end point was significantly lower at
1 year in patients treated with homocysteine-lowering therapy (15.4%
vs 22.8%; relative risk [RR], 0.68; 95% confidence interval [CI], 0.48-0.96;
P =.03), primarily due to a reduced rate of target lesion revascularization
(9.9% vs 16.0%; RR, 0.62; 95% CI, 0.40-0.97; P =.03). A nonsignificant
trend was seen toward fewer deaths (1.5% vs 2.8%; RR, 0.54; 95% CI,
0.16-1.70; P =.27) and nonfatal myocardial infarctions (2.6% vs 4.3%;
RR, 0.60; 95% CI, 0.24-1.51; P =.27) with homocysteine-lowering therapy.
These findings remained unchanged after adjustment for potential confounders.
CONCLUSION: Homocysteine-lowering therapy with folic acid, vitamin
B(12), and vitamin B(6) significantly decreases the incidence of major
adverse events after percutaneous coronary intervention. |
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Homocysteine and risk of ischemic heart disease and stroke: a meta-analysis.
Homocysteine Studies Collaboration. |
JAMA 2002 Oct 23-30;288(16):2015-22 Abstract quote
CONTEXT: It has been suggested that total blood homocysteine concentrations
are associated with the risk of ischemic heart disease (IHD) and stroke.
OBJECTIVE: To assess the relationship of homocysteine concentrations
with vascular disease risk.
DATA SOURCES: MEDLINE was searched for articles published from January
1966 to January 1999. Relevant studies were identified by systematic
searches of the literature for all reported observational studies of
associations between IHD or stroke risk and homocysteine concentrations.
Additional studies were identified by a hand search of references of
original articles or review articles and by personal communication with
relevant investigators.
STUDY SELECTION: Studies were included if they had data available by
January 1999 on total blood homocysteine concentrations, sex, and age
at event. Studies were excluded if they measured only blood concentrations
of free homocysteine or of homocysteine after a methionine-loading test
or if relevant clinical data were unavailable or incomplete.
DATA EXTRACTION: Data from 30 prospective or retrospective studies
involving a total of 5073 IHD events and 1113 stroke events were included
in a meta-analysis of individual participant data, with allowance made
for differences between studies, for confounding by known cardiovascular
risk factors, and for regression dilution bias. Combined odds ratios
(ORs) for the association of IHD and stroke with blood homocysteine
concentrations were obtained by using conditional logistic regression.
DATA SYNTHESIS: Stronger associations were observed in retrospective
studies of homocysteine measured in blood collected after the onset
of disease than in prospective studies among individuals who had no
history of cardiovascular disease when blood was collected. After adjustment
for known cardiovascular risk factors and regression dilution bias in
the prospective studies, a 25% lower usual (corrected for regression
dilution bias) homocysteine level (about 3 micromol/L [0.41 mg/L]) was
associated with an 11% (OR, 0.89; 95% confidence interval [CI], 0.83-0.96)
lower IHD risk and 19% (OR, 0.81; 95% CI, 0.69-0.95) lower stroke risk.
CONCLUSIONS: This meta-analysis of observational studies suggests that
elevated homocysteine is at most a modest independent predictor of IHD
and stroke risk in healthy populations. Studies of the impact on disease
risk of genetic variants that affect blood homocysteine concentrations
will help determine whether homocysteine is causally related to vascular
disease, as may large randomized trials of the effects on IHD and stroke
of vitamin supplementation to lower blood homocysteine concentrations. |
