Monday July 9 2007
A new blood test that measures the levels of a protein called myeloperoxidase (MPO), could identify healthy people who are at risk of a heart attack within the next eight years, The Times reported on July 7 2007. The newspaper said that people with significantly more MPO in the blood than average were about 1½ times more likely to have a heart attack or heart disease within the next eight years.
The Times stated: “A new blood test could alert apparently healthy people to the risk of suffering a heart attack in up to eight years’ time.”
This study does indicate that MPO levels may prove to be a useful marker for risk of heart disease in healthy people. However, the researchers found that adding an assessment of MPO levels to an assessment of the traditional risk factors for heart disease did not significantly improve predictive ability.
More research needs to be carried out on the possible benefits of adding this test to the existing array of tests for heart disease.
Where did the story come from?
The story was based on a study published in the peer reviewed Journal of the American College of Cardiology. The study was conducted by Dr Matthijs Boekholdt and colleagues at centres in Holland, US, and the UK.
The people who took part in this study were all from Norfolk, England. The study was funded by Wyeth Research in Cambridge, Massachusetts, the Medical Research Council UK, Cancer Research UK, the European Union, the Stroke Association, the British Heart Foundation, and the Wellcome Trust. One of the authors is named as a co-inventor on patents that have been submitted relating to the use of MPO as a marker for cardiovascular disease.
What kind of scientific study was it?
Almost 3,400 healthy volunteers with no history of heart attacks or strokes were enrolled in the study during the 1990s. During enrolment, a sample of their blood was taken and stored at very low temperatures (-80C/-112F) to be used for future analysis. Over an eight-year period, researchers monitored the volunteers and identified those who had been hospitalised for, or who had died from, coronary artery disease (CAD) .The researchers found that 1,138 volunteers experienced CAD events during the eight-year follow-up compared with 2,237 volunteers who did not experience any CAD events.
The blood samples taken at the start of the study were then measured for their levels of the protein MPO and the results from the two groups were compared. The people testing the blood samples were blinded.
This was a case-control study which was part of a larger study of risk factors for heart disease (the European Prospective Investigation Into Cancer and Nutrition – Norfolk).
What were the results of the study?
The study found that levels of the protein MPO in the original blood tests were higher in the people who later developed heart disease than in those who did not. After taking into account the volunteers’ known risk factors for heart disease (such as blood pressure, cholesterol levels, body mass index, smoking, and diabetes), the researchers found that the people with the highest levels of MPO (those within the top 25%) were 36% more likely to develop heart disease compared to those with the lowest levels (the bottom 25%).
If the results were adjusted for levels of another protein related to inflammation, C-reactive protein, the increase in odds of developing heart disease with MPO was no longer significant, indicating that MPO did not predict heart disease independently of C-reactive protein.
What interpretations did the researchers draw from these results?
The researchers concluded that, in apparently healthy people, raised MPO levels are associated with an increased risk of heart disease in the future.
What does the NHS Knowledge Service make of this study?
This study does indicate that MPO levels may prove to be a useful marker for risk of heart disease in healthy people. It is part of a large, well-designed study, but these results should be considered only as preliminary findings and more research needs to be carried out before any practical use can be made of them.
Important limitations of the study include the facts that:
- Levels of MPO and other blood components were tested in only one sample for each volunteer, were not taken at standard times of the day, and were stored for a prolonged period before testing. The results of the study could therefore be affected by these factors.
- The identification of the people who later developed heart disease was based on checking records from death certificates and hospital admission data; this could mean that cases might have been missed if they were recorded inaccurately.
- Analyses found that adding an assessment of MPO levels to an assessment of the traditional risk factors for heart disease did not significantly improve predictive ability. This suggests that assessing MPO levels may not add much to the methods of assessment that are already available.
Sir Muir Gray adds ...
Individual risk markers are now of little interest because we need to know how much value a new marker will add to the set of markers used at present. The next step needs to be modelling of the effects that the addition of this marker might have on the benefits and costs of the current approach.