If, as Socrates posited, the unexamined life is not
worth living, doubtless some wag has also noted that the overexamined
life is a bore to everyone. Into the breach steps Robert Christenson,
PhD, who urges pathologists to reconsider, but not wholly trash, what
they thought they knew about biochemical markers of long-term cardiovascular
Leaving few markers unturned, Dr. Christenson systematically
examined the doubts, and data, surrounding current biomarkers, in an AACC
conference, held last spring, on cardiac risk assessment, diagnosis, and
management. In his talk, Dr. Christenson took an evidence-based poke at
markers of primary prevention.
Cystatin C is a worthy example of how Dr. Christenson
approaches matters. The marker may be unfamiliar to some. "If you haven't
heard much about this protein, you probably ought to Google it," joked
For those who've yet to do a search, Dr. Christenson
provided a few basics. Cystatin C is a single-chain basic protein, nonglycosylated,
with a molecular mass of 13,360 kD. It inhibits cysteine proteases and
is synthesized by all nucleated cells. It demonstrates a constant production
rate typical of so-called housekeeping genes—in other words, it's
not influenced by acute-phase reactions. It's cleared by free glomerular
filtration. Tubular reabsorption and rapid breakdown occur by proximal
tubular cells; it is not secreted by tubules or eliminated by any extra-renal
route. Neither muscle mass, nor food intake, nor body surface influence
It's this last feature that makes cystatin C particularly
intriguing, said Dr. Christenson, and gives it a leg up on creatinine.
"But," he cautioned, "it's more like a $5 test, rather than a 50-cent
One of the key investigations that put cystatin C on
the map, said Dr. Christenson, was measurement in the cohort of the Cardiovascular
Health Study (Shlipak MG, et al. N
Engl J Med. 2005;352:2049-2060), which compared creatinine and
cystatin C levels as predictors of mortality, from both cardiovascular
causes and all causes, in the elderly. Cystatin C was found to be a substantially
stronger predictor of risk of death and cardiovascular events. The researchers
identified participants as being at low, intermediate, and high risk using
cystatin C levels of <1 mg/L, 1-1.28 mg/L, and >1.29 mg/L, respectively.
Moreover, the study's authors note, cystatin C levels
defined not only the top 20 percent of elderly people in the CHS cohort
with a substantially higher risk of death, but also a large subgroup (the
lowest 40 percent) at below-average risk of death. Cystatin C is likely
associated with all-cause and cardiovascular mortality, Dr. Christenson
said, and this information should remind physicians that patients with
renal insufficiency are at higher risk for cardiovascular events. Some
investigators suggest that cystatin C even has a direct (that is, not
exclusively renal) impact on cardiovascular risk, he added.
The point of his discussion was not to spur a run on
cystatin C. A good biomarker is only as good as what it brings to the
bigger picture—think group photo, not head shots. How do established
markers and emerging markers fit in with the other tools clinicians use?
What is the right way to analyze them? In addition to searching for new
markers, are there better ways of thinking about traditional ones?
The last 20 years have shown that cardiac incidents
are motley events, involving the unhappy confluence of inflammation, impaired
endothelial function, impaired plaque stabilization, formation of reactive
species such as oxygen-free radicals, as well as the crowning event of
thrombus formation. Thinking about markers in terms of their varied physiological
processes probably is helpful, he said, and has given rise to multimarker
It might also be helpful for lab folks to step out
of their domain, at least metaphorically, and think about how biomarkers
fit into clinicians' worldview and their necessarily wider consideration
In his own lab at the University of Maryland School
of Medicine, Baltimore, where he's professor of pathology and professor
of medical and research technology, Dr. Christenson said he himself failed
to fully realize the high risk to patients with impaired renal function,
in part because the Framingham Risk Score does not contain a term for
impaired glomerular filtration rate.
GFR is part of pathology's dialect, but FRS may not
be. The Framingham Risk Score is a prediction tool for estimating 10-year
risk of myocardial infarction or cardiovascular death. It considers factors
both uncontrollable (age—"I wish we could do something about age,"
Dr. Christenson said with a laugh—and HDL) and controllable (smoking,
blood pressure to some extent, LDL). With these cards on the table, the
question changes slightly: What more can biomarkers do to add to the current
ability to assess cardiac risk? "What we have to remember is the additive
effect of what any biomarker brings. It can't be looked at by itself—it
has to be looked at in the context of the way the clinician is going to
The FRS remains the recommended starting point for
exploring cardiac risk (http://hp2010.nhlbihin.net/atpiii/calculator.asp?usertype=prof),
said Dr. Christenson, who performed the calculation using himself as a
model ("a fella in his late 40s—OK, 53," he conceded) to arrive
at a risk of five percent. By making additional, hypothetical adjustments,
he bumped up his risk—if he were a smoker, for example, he'd more
than double his risk, possibly making him a candidate for treatment with
"Any biomarker that we come up with must help with
this risk," he said. "If it doesn't increase what we already can do in
the clinics, then the marker is of modest value at best." Viewed from
the standpoint of physiology, markers must be able to help clinicians
determine which patients might be progressing toward atherosclerosis,
plaque erosion and rupture, activation of the coagulation cascade, and
the like. On the backside, markers might be able to help clinicians sort
through various outcomes, such as coronary artery disease, peripheral
artery disease, AMI, stroke, and heart failure.
Inflammation is certainly worth thinking about. It's
fundamental in the pathogenesis of atherosclerosis, and it predicts risk
of coronary heart disease in primary and secondary prevention settings,
as well as the efficacy of some medications. There's no shortage of markers
for monitoring inflammation, but how do we know which ones are best? "Why
is it that you've seen hs-CRP on the cover of Time magazine?"
but not fibrinogen or SAA, Dr. Christenson asked. "Part of the answer
lies in stability of the biomarker, standardization, and, in short, our
ability to report on the marker."
It's also partly because high-sensitivity C-reactive
protein puts laboratorians on solid footing with clinicians. "We have
to walk the walk and talk the talk," he said. That means using the same
criteria and grading evidence clinicians rely on. These are derived from
American College of Cardiology/American Heart Association classifications
and summary of indications (Pearson TA, et al. Circulation.
2003;107:499-511). Classification I is the sturdiest, backed by evidence/general
agreement that a given procedure or treatment is useful and effective,
while level A offers the best weight of evidence, girded by data derived
from multiple randomized clinical trials involving large numbers of patients.
The Framingham Risk Score, for example, is a classification: 1, level
of evidence: A. This leaves little room for quibbling.
One reason for hs-CRP's popularity is slap-your-head
simple: It's easy to measure. "We've done a great job in achieving a level
of standardizing the measurements," Dr. Christenson says. Because of this,
it's easy to talk about hs-CRP with clinicians with a green, yellow, and
red analogy. Levels below 1 mg/L are consistent with low risk; between
1-3 mg/L, average risk; and above 3 mg/L, high risk. Very high concentrations,
greater than 10 mg/L, suggest an inflammatory process and require re-measurement
at a later time.
A review of assays of inflammatory markers from the
aforementioned Circulation article looked at hs-CRP as well as
soluble adhesion molecules (for example, E- and P- selectin, intracellular
adhesion molecule-1, and vascular cell adhesion molecule-1), various cytokines
(for example, interleukin-1 -6, -8, and -10, and tumor necrosis factor-alpha),
and the acute-phase reactants fibrinogen and serum amyloid A. Only hs-CRP
displayed all four hallmarks for abetting clinical use: stability, many
available assays, available standards from the World Health Organization,
and good interassay precision (it has a CV of <10 percent). Like the Framingham
criteria, hs-CRP enjoys an ACC/AHA classification of 1 and a level of
evidence of A. It has, said Dr. Christenson, the most appropriate analyte
and assay characteristics of the current inflammation markers for clinical
use. "There's a lot of good evidence," he said.
The next step is to evaluate markers in terms of treatment.
Though hs-CRP indicates risk, the evidence for its use in directing further
evaluation and therapy in the primary prevention of cardiovascular disease
is dodgy—in terms of the ACC/AHA guidelines, it's ranked as class
IIa, meaning the weight of evidence/opinion is in favor of usefulness/efficacy.
That may sound good, but in guideline-speak, it's far from a ringing endorsement.
A study from the Annals
of Internal Medicine (Cook NR, et al. 2006;145:21-29) shows what
even a good test is up against. It looked at the impact of hs-CRP on the
risk classification of nearly 27,000 nondiabetic women. Nearly 88 percent
of the women had a baseline 10-year FRS risk of less than five percent.
In that group, there's no need to measure hs-CRP (classification: 1; level
of evidence: A). In this group, said Dr. Christenson, risk can't sink
any lower. "So getting an hs-CRP measurement to lower their risk is absurd."
In the highest risk group (women with a baseline FRS risk of 20 percent
or higher), adding hs-CRP is also unlikely to add useful information because
treatment for that group will most likely be implemented regardless of
the result. "You're already at high risk and a strong candidate for treatment,
so what good is the hs-CRP going to be?" Dr. Christenson asked.
Dr. Christenson suggested hs-CRP adds value for the
three percent of women whose baseline FRS risk falls into the 10 to less
than 20 percent range, where clinicians might be unsure about prescribing
preventive therapies such as a statin. In this group, adding hs-CRP might
reclassify patients into either a higher or lower risk category, defogging
things a bit (1/A). About 13.7 percent would be reclassified downward,
and their risk, after recalculation with hs-CRP, would be about 6.8 percent.
Five percent would be reclassified at higher risk, with a recalculated
risk of nearly 19.9 percent. For clinicians and patients wavering about
treatment, recasting the risk may make their decision about whether to
treat more clear-cut.
If the baseline FRS risk is between five and 10 percent,
close to 10 percent of patients might be reclassified to a higher risk
group if an hs-CRP measurement is added. It's not clear if moving women
into this group has clinical benefit, especially in terms of longer-term,
lifetime risk prediction (IIa/B). The authors suggested, however, that
women with FRS risk between five and 20 percent might benefit from adding
Following the money trail is also a good way to evaluate
what new markers might bring to the table, said Dr. Christenson, who not
only walked the walk and talked the talk, but gave a decent impression
of an actuary to boot.
He started by reviewing the estimated cost-effectiveness
of various LDL-lowering therapies. If the 10-year FRS risk is 35 percent,
even a higher-end drug—one costing, say, $3/day, or roughly $1,000/year—is
"a pretty good deal," he says, noting the cost-effectiveness (cost per
quality-adjusted life years gained) of this strategy is $10,000.
In the 25 percent FRS risk category, the cost per QALY
gained is still a pretty easy decision: $25,000 for the high-end drug.
What about a 10 percent FRS? "Would you treat all those
patients?" Dr. Christenson asked. If the treatment runs $3/day, "you have
to be prepared to make some decisions," he said, noting this carries a
tab of $100,000 cost per QALY gained. At five percent, the $3/day drug
translates into $200,000 cost per QALY gained. "You can see where this
is going," he said.
The 10 percent figure, he added, is considered the
financial tipping point, and the reason why intermediate risk is defined
as being 10-20 percent. "So when we look at these emerging markers, cost
must be a very important consideration."
Maybe measuring hs-CRP will motivate patients to quit
smoking, he acknowledged. "But nobody has done a trial" to see if this
is true. The evidence suggesting a link between hs-CRP levels and lifestyle
changes is scant (classification: IIb, level of evidence: C).
How did hs-CRP hold up to Dr. Christenson's scrutiny?
Perhaps not as well as Time magazine would have it.
It has predictive power, but is not proven to be directly
causative, Dr. Christenson said. It's influenced by body weight and highly
correlated with metabolic syndrome. The assay's technical characteristics
are good, but—and this is important—it's not yet integrated
into the FRS assessment. At this point, he said, its clinical utility
appears to be as an add-on risk marker.
And other biomarkers?
Homocysteine's star may be dimming a bit, said Dr.
Christenson. It has positive association with CVD risk, though it's not
integrated into global risk assessment. It can be reduced by folate and
vitamins B12 and B6, and clinically, high levels are treated with vitamins.
But, Dr. Christenson noted, the vitamin trials have generally been negative.
This is because lowering homocysteine by 25 percent—the figure used
in most analyses—doesn't lower risk, according to a meta-analysis
published in JAMA
(2002;288:2015-2022). "If you lower it and there is no improvement in
outcome, it obviously can't be used as a target for therapy."
Despite its tangible link to ischemic heart disease,
he said, homocysteine doesn't add much to the FRS, and measuring it isn't
warranted for primary prevention and assessment of cardiovascular risk
(III/C). Homocysteine levels in high-risk patients may have fairly modest
ability to predict future cardiovascular events in those with multiple
risk factors, such as renal insufficiency, hypertension, or metabolic
While admiring the heart's stellar pumping attributes,
Dr. Christenson admits to a fondness for its role as a hormone-producing
organ. BNP and NT-proBNP affect natriuretic resistance, with a resulting
impact on kidney as well as basal dilation. "I think of this as a marker
of heart stress," such as increased ventricular load, he said. "This has
an advantage over hs-CRP, although it's a whole different mechanism."
Dr. Christenson's enthusiasm comes, in part, from a
"wonderful" prospective study published in the New
England Journal of Medicine (Wang TJ, et al. 2004;350:655-663)
that looked at 3,346 patients without heart failure. The patients who
fell into the highest third of BNP scores still had relatively low baseline
scores: >12.8 pg/mL for men, and >15.8 pg/mL for women.
Noted Dr. Christenson: "You have to be prepared, if you use this in primary
prevention, that very low values are what we're going to use for our risk
stratification decisions." Plasma NPs predicted risk of death and cardiovascular
events after adjustment for traditional risk factors, with excess risk
apparent at levels well below current thresholds used to diagnose heart
failure, he said.
Citing several other studies, he called NT-proBNP and
BNP "death markers." Virtually all studies have shown that if you have
high values, you're at increased risk.
Is the evidence all in? Certainly not. The ACC/AHA
scorecard rates the markers III/B: Benefits of therapy based on marker
measurements are uncertain, and measurement for primary prevention and
assessment of cardiovascular risk is not advised. But, said Dr. Christenson,
"I think it offers another piece of that physiological evidence, along
with inflammation, and maybe unstable plaque, that there's heart stress
Another "old" marker came in for a new look—microalbumin,
which, says Dr. Christenson, is another powerful risk factor that reflects
a renal component and perhaps increased pressure at the level of the glomerulus.
A number of studies make that point. But, he warned, labs still don't
have a good handle on the standardization, appropriate units for measurement—the
studies bounce between milligrams/millimole and micrograms/minute—and
other measurement issues.
To round out the picture, Dr. Christenson turned to
another study by Thomas Wang, MD (N
Engl J Med. 2006;355:2631-2639), this one looking at multiple
biomarkers for predicting first major cardiovascular events and death.
While the study did not have primary prevention as its focus, Dr. Christenson
suggested this study was still worth a close look.
Ideally, he said, it would be great if markers could
be used to help steer people into low-, intermediate-, and high-risk groups.
"Wouldn't that be a useful addition to the Framingham score?" he asked.
Alas, they couldn't pull it off, at least not in this
study, which looked at 10 emerging biomarkers: hs-CRP, BNP, N-terminal
pro-atrial natriuretic peptide, aldosterone, renin, fibrinogen, D-dimer,
plasminogen-activator inhibitor type 1, homocysteine, and urinary albumin-to-creatinine
Adding the multimarker approach to what clinicians
already use was of little benefit in predicting death. The ROC curve area
(also referred to in the study as C statistic) for age, sex, and conventional
risk factors as predictors was .80; for age, sex, and multimarkers, the
ROC curve area was .79. Adding all predictors produced little added benefit—the
ROC curve area was .82.
Doing the same type of analysis for major CV events
has yielded similar, slightly disappointing results. Age, sex, and conventional
risk factors translated into an ROC curve area of .76. The ROC curve area
for age, sex, and multimarkers was .70. Using all predictors was .77—again,
adding multimarkers made only a small dent.
Numbers such as these explain the muted enthusiasm
for cardiovascular biomarkers, Dr. Christenson said. But, he added, this
approach—ability to shift the ROC curve—"is a fair way to
look at markers. I think there is a science right now that I hope some
of us are turning to."
"Certainly the ROC curve is a very high mark to jump,
and it's a tough one," he continued. In this study, the markers added
moderate value to the standard risk factors, the researchers concluded,
a statement Dr. Christenson called "pretty kind," given that the markers
had only a small increase in the area under the ROC curve. "Even a great
enthusiast like me about these markers has to be cautious after examining
this sort of study."
Where does this leave laboratorians? In pursuit of
novel biomarkers and perhaps with a handful of questions, suggested by
Dr. Christenson as a way to evaluate them:
- Is it predictive of risk?
- Is it an independent risk factor?
- Can the emerging risk factor be used as an add-on
global risk assessment?
- Can it be incorporated into a global risk-assessment
algorithm? If not, why not?
- Is the marker an innocent bystander, or is it involved
in physiologic process? If it's involved, could it be a target of therapy?
- What is the cost? What is the availability of testing?
Until these questions are answered, perhaps no emerging
marker should earn an answer of "yes" to another question: Does it belong
on the cover of Time?
Karen Titus is CAP TODAY
contributing editor and co-managing editor.