Pierce, Janet D. DSN, APRN, CCRN; McCabe, Shannon BS; White, Nicole BSN, RN, CCRN; Clancy, Richard L. PhD
Historically, health care professionals have used vital signs like blood pressure and heart rate as markers to assess health status and evaluate how diseases respond to therapeutic interventions. More recently, biologic markers or “biomarkers” have been used to evaluate cellular and organ function along the continuum of health and illness.
The importance of biomarkers continues to grow in all areas of clinical practice and, whether to predict, diagnose, or monitor disease, biomarkers are useful in every step of patient care. While disease symptoms are subjective, biomarkers provide an objective, measurable way to characterize disease. They can often be measured by analyzing blood or urine samples, helping clinicians avoid complex invasive procedures. As biomarker research is being translated into clinical practice, all health care professionals should be aware of what biomarkers are, how they are used, and the effects their use can have on patient outcomes.
The Biomarkers Definitions Working Group, an affiliate of the National Institutes of Health, defines a biomarker as “a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention.”1 More specifically, the National Cancer Institute defines a biomarker as “a biological molecule found in blood, other body fluids, or tissues that is a sign of a normal or abnormal process or of a condition or disease. A biomarker may be used to see how well the body responds to a treatment for a disease or condition.”2 Two British researchers writing on the use of biomarkers in detecting and treating acute myocardial infarction (AMI) offer the following: “A good biomarker is something that is easily measured and can be used as a surrogate marker for disease and its severity. For instance, blood sugar can be used to diagnose diabetes whilst glycosylated hemoglobin (HbA1c) monitors blood sugar control.”3 Indeed, biomarkers are used to help diagnose disease, classify the disease process in terms of stage or severity, and monitor and assess the progress of treatment.4
The purpose of this article is to review common biomarkers used in diagnosing and treating sepsis (such as procalcitonin [PCT] and C-reactive protein [CRP]), AMI (such as troponin), asthma (such as exhaled nitric oxide [eNO]), and oxidative stress (such as glutathione [GSH] and malondialdehyde [MDA]). We also review examples of predictive biomarkers: genes, molecules, and cells that act like biomarkers and are currently advancing cancer diagnosis and treatment; these include the breast cancer 1 gene (BRCA1), cytochrome P-450 2D6 (CYP2D6), tumor cells, and the human epidermal growth factor receptor 2 (HER2) protein.
Biomarkers can be classified in functional terms as predictive, diagnostic, prognostic, staging, or pharmacodynamic (see Figure 1). Some biomarkers, such as CRP, may be used for more than one or all of these purposes. Predictive biomarkers are used to indicate the potential threat of disease. Diagnostic biomarkers assist in pinpointing the disease or ailment. Prognostic biomarkers are used as a guide for treatment, a report card for progress, and a tool to ensure the patient is receiving the appropriate care and management. Staging biomarkers are often used to classify the extent of diseases. Pharmacodynamic biomarkers are used to gauge or predict the body's response to drug intervention in order to identify whether a specific drug will be effective in a particular patient and are often used in clinical trials of new drugs.5
Sensitivity and specificity. Tests of sensitivity and specificity are commonly used to evaluate the performance of a biomarker. Sensitivity refers to the percentage of individuals among those with a particular disease or condition who test positive for the biomarker associated with the disease or condition.6 Specificity refers to the percentage of individuals among those who do not have a particular disease or condition who test negative for the biomarker associated with the disease or condition. A superior biomarker would have both high sensitivity and high specificity; however, biomarkers with high sensitivity usually have low specificity.
Biomarkers are used in many areas including research, clinical assessment, and intervention to determine whether a characteristic is normal, pathogenic, or a pharmacologic response. To be of use in the treatment and intervention process, biomarkers should offer an early detection of disease and preferably be quickly obtainable, economical, reliable, and easily interpretable.7 A superior biomarker should also reflect the pathogenic process of the disease and respond to interventions; a biomarker that aids only in identification of a disease is of less value. More importantly, biomarkers for specific diseases should be consistent across different populations.
As a result of extensive research and clinical experience, the importance of biomarkers in the diagnosis and treatment of many diseases has grown; indeed, the use of biomarkers has become part of the standard of care for many diseases. Improvements in the use of biomarkers to diagnose and classify disease and monitor and assess treatment should result in more efficient management of disease and improve individualization of care.
Sepsis is one of the leading causes of death in the ICU; therefore, sepsis biomarker research and development is a high priority, and there is still controversy about the optimal sepsis biomarker. According to the Centers for Disease Control and Prevention, hospitalizations in the United States for septicemia or sepsis as the principal diagnosis increased from 326,000 in 2000 to 727,000 in 2008, and the rate of these hospitalizations more than doubled from 11.6 to 24 per 10,000 population during this period.8 Hospitalizations with septicemia or sepsis as a principal or secondary diagnosis (including patients who had the condition while admitted for another diagnosis and those who acquired it nosocomially) increased from 621,000 in 2000 to 1,141,000 in 2008. Worldwide, there are approximately 18 million new cases of sepsis each year, and mortality rates range from 30% to 60%.9
Clinicians often have difficulty diagnosing sepsis, as it shares many clinical symptoms with other conditions that arise in ICU patients. During lengthy hospitalizations, critically ill patients often develop systemic inflammatory response syndrome (SIRS), the first of four levels of sepsis as defined by the Society of Critical Care Medicine (the others are sepsis, severe sepsis, and septic shock). Symptoms of SIRS include abnormal heart rate, body temperature, respiratory rate, and white blood cell count.10 Pinpointing the source of a septic infection is important in determining an ICU patient's clinical outcome. One challenge is that many patients who have prolonged stays in the ICU often exhibit fluctuating symptoms that can be caused by hypoxia, drug therapy, SIRS, infections, and inadequate tissue perfusion, among other factors.11
In the diagnosis of blood infections, blood cultures have been shown to lack sensitivity, specificity, or both. Studies have shown that two blood cultures performed in a 24-hour period detect only 80% to 90% of bloodstream infections in adults.12, 13 Given the ambiguities of clinical signs for diagnosis and severity, the use of biomarkers can greatly improve diagnostic accuracy and prognostic assessment.14
CRP and PCT are the most commonly used biomarkers for sepsis and other bacterial infections. CRP is a protein synthesized by the liver and released into the bloodstream in response to inflammation. However, the use of CRP as a biomarker for sepsis and other conditions has been criticized because it lacks specificity and is inadequate in monitoring the progression of an infection.15 Many other diseases and conditions, including burns and trauma, can also cause an elevation of CRP, thus it is not specific to sepsis. Also, CRP levels are high in the early stages of infection but generally do not increase with a subsequent increase in severity.16
Much evidence has pointed to PCT as the biomarker of choice in sepsis.14, 15 PCT is a peptide precursor of calcitonin, which maintains calcium homeostasis; cells in the thyroid, the lung, and the intestine can produce and release PCT into the blood in response to either an increase in serum calcium level (from the thyroid) or bacterial infection (from the lung and intestine). PCT follows a similar expression pathway to cytokines and in the cascade of events is induced by tumor necrosis factor-α and interleukin-6, which are inflammatory indicators.17 PCT is elevated in the presence of a bacterial infection.18
PCT has value as a diagnostic biomarker because it increases at an early stage of infection and then decreases rapidly when the infection is controlled by the immune system or antibiotics.14 Kinetically, PCT reacts more quickly than CRP and testing for it can produce a diagnosis of sepsis 24 to 48 hours sooner.16 PCT concentration also correlates with the extent and severity of infection and is superior to CRP as a guide for antibiotic therapy. Furthermore, studies have shown that when PCT was used as a marker, septic ICU patients had shorter courses of antibiotic therapy and stays in the ICU.19 However, PCT concentration has been shown to increase in events like stress and trauma, even though the increase is moderate and temporary. Therefore, sequential measurements of PCT are recommended in order to properly evaluate the course of treatment.20, 21
MYOCARDIAL INFARCTION BIOMARKERS
Heart disease is the leading cause of death in the United States; in 2009, it accounted for nearly 600,000 deaths.22 Heart disease is also the leading cause of death worldwide.23 The risk of death and disability as the result of AMI is at its peak within the first few hours of AMI onset. Therefore, researchers and clinicians have long sought to identify biomarkers that can be used reliably to diagnose and outline a treatment plan for AMI.
The cardiac biomarker troponin plays an important role in the redefinition of AMI put forth in 2000 by the American College of Cardiology and the European Society of Cardiology.24, 25 A protein that aids in muscle contraction, troponin is found in both skeletal and cardiac muscle. There are three isoforms of troponin (TnC, TnI, and TnT); two of these are highly specific to cardiac muscle (cTnI and cTnT) and are released into the blood in the event of cardiomyocyte necrosis (cardiac cell death), such as occurs in AMI.3, 26 Mills and colleagues conducted a study of patients with suspected acute coronary syndrome, a group of conditions resulting from acute myocardial ischemia (insufficient blood flow to the heart) that ranges from unstable angina to myocardial infarction. The researchers found that troponin testing increased the rate of AMI diagnosis and identification of high-risk patients.27 A 2009 study by Keller and colleagues showed that cTnI improves early diagnosis of AMI better than cTnT regardless of time of chest-pain onset.28
Troponin is the most sensitive and specific biomarker for myocardial damage. Other biomarkers such as myoglobin and creatine kinase-MB (CK-MB) are also used in conjunction with cTnI in diagnosing AMI. However, studies have shown that cTnI is a more suitable diagnostic biomarker for this purpose, because myoglobin and CK-MB are not specific to cardiac muscle.29 But much depends on timing: troponin levels typically peak 12 hours after the infarct and may remain elevated for 10 days or more.3 Therefore, if the myocardial injury has occurred within two hours, it would be more appropriate to test for myoglobin. Because of troponin levels' delayed peak, it is important to repeat troponin assays upon admission and multiple times thereafter.
A diagnosis of AMI can be made when, in combination with other measures or observations, at least one value of cardiac troponin among several ongoing measurements is above the 99th percentile of the upper reference limit; in addition, there should be clinical signs of ischemia, such as electrocardiographic changes and “imaging evidence of new loss of viable myocardium or new regional wall motion abnormality.”30 Thus, troponin has emerged as the preferred biomarker for AMI.
Transforming growth factor beta receptor 1 is proposed as a prognostic biomarker after AMI, but its use needs further investigation.31 Troponin assays continue to be utilized and researched in order to increase sensitivity and refine the use of troponin as the biomarker of choice in AMI.
Asthma is a chronic respiratory disorder characterized by airway inflammation.32 Affecting approximately 300 million people worldwide and almost 26 million in the United States, asthma is one of the most prevalent chronic illnesses and contributes to an estimated 250,000 deaths per year globally; there were almost 3,400 asthma-related deaths in the United States in 2009.33, 34
The most widely investigated and promising biomarker for asthma is eNO.32 NO is synthesized from the amino acid L-arginine by the enzyme NO synthase and is elevated in the lungs in response to inflammation. Unlike typical invasive procedures for measuring airway inflammation such as bronchoscopy and bronchoalveolar lavage, measuring eNO is convenient, quick, and noninvasive.33 However, levels of eNO are affected by age, height, weight, gender, race, and exhaled flow rates.35
Many patients manage asthma with corticosteroid treatments. Current clinical practice lacks an inflammatory biomarker capable of predicting the effectiveness of corticosteroid treatment in asthma.33 The problem with finding an ideal inflammatory biomarker is the differing and sometimes unique responses observed in patients using corticosteroid therapy. Clinical trials use eNO as a biomarker to predict the outcomes of new asthmatic therapies. The use of eNO in evaluating the effectiveness of over-the-counter medications merits investigation.36
Some investigators have suggested that, because of the complex nature of asthma, multiple biomarkers are needed for early diagnosis and management.37 The need for more than one biomarker is supported by the heterogeneous nature of the antiinflammatory therapeutic response in asthma patients mentioned above. Despite that eNO is the most widely used and investigated biomarker for asthma, its clinical utility needs to be explored further, along with the inflammatory patterns and variation in therapeutic response among patients, in order to find the ideal biomarker (or combination of biomarkers) for asthma diagnosis and management.
OXIDATIVE STRESS BIOMARKERS
Oxidative stress is a state of metabolic imbalance in which the production of reactive oxygen species (ROS) exceeds their neutralization by antioxidants.38 ROS are free radicals, including hydroxyl, hydrogen peroxide, and the superoxide anion, that are formed in the course of metabolizing oxygen for energy production. Excessive free radicals can cause damage to DNA, lipids, and proteins, leading to apoptosis and necrosis. These phenomena contribute to the pathogenesis of many disease processes such as amyotrophic lateral sclerosis, Huntington's disease, Parkinson's disease, retinal degenerative disorders, atherosclerosis, cancer, and shock.39
Free radicals are short-lived species with half-lives of less than one second.39 Therefore, more stable molecular products of oxidative stress, including lipid peroxidation end products, oxidized proteins, or antioxidants, have become increasingly important as biomarkers for measuring this condition.40 Lipid peroxidation occurs when a free radical captures an electron, typically from a polyunsaturated fatty acid, resulting in impairment of various cell membrane functions. MDA, one of the by-products of lipid peroxidation, is used as a biomarker to measure the degree of oxidative stress and has been shown to be mutagenic and carcinogenic.
Another example of an oxidative stress biomarker is GSH, the major endogenous antioxidant in the cell. The measurement of GSH can provide fundamental information on the extent of oxidative stress–related damage that can lead to various diseases.41
Because various pathways lead to oxidative stress, there are many ways to measure the condition, and it's hard to know which biomarker or combination of biomarkers is optimal. Additionally, many of these biomarkers lack the sensitivity and specificity needed for validation, and measuring them often requires invasive and specialized techniques. It should also be noted that oxidative stress biomarkers have very low basal concentrations.
CANCER BIOMARKERS AND GENOTYPING
Cancer is a complex disease, and caring for patients with cancer requires awareness of current and emerging therapies. There are multiple clinical uses for biomarkers in cancer, including in staging, monitoring, screening, therapy selection, diagnosis, and prognosis.42 For example, the BRCA1 gene is used to predict risk and survival of breast cancer. The breast cancer genes can act like biomarkers: they can accurately predict clinical outcomes and be used as a bridge between basic research and clinical applications.43
Biomarkers are used in breast cancer patients to predict responsiveness to prescribed therapies. For example, variations in tamoxifen metabolizers such as CYP2D6 can be used as biomarkers to predict response to tamoxifen treatment. CYP2D6 converts tamoxifen to an anticancer agent. Thus, testing for variations in CYP2D6 can guide the use of tamoxifen in therapy selection for breast cancer patients.44
Another important area for genotyping in breast cancer management is identification of the HER2 gene. Activation of HER2 results in cell proliferation and resistance to apoptosis, leading to rapid tumor development and progression.45 Approximately 25% of breast cancers are classified as HER2-positive, which denotes an aggressive phenotype.45-47
Gene markers like HER2 are used to guide clinicians in chemotherapy selection. For example, patients with HER2-positive breast cancer had a 50% reduction in the early risk of recurrence and a significant overall survival benefit with trastuzumab treatment.48 However, patients who do not exhibit an amplification of the HER2 gene may not benefit from HER2-targeted treatments.47
More recently, advancements have been made in cancer-related biomarkers for both screening and monitoring, including a blood test that can isolate a single circulating tumor cell among a billion healthy ones in the blood of a cancer patient.49, 50 This biomarker can be used to estimate both tumor shrinkage and the success of certain treatments. The sensitivity and noninvasiveness clearly make it superior to less-sensitive computed tomographic scans.
OTHER COMMON BIOMARKERS
There are many other biomarkers that are commonly applied in practice (see Table 1 for examples); and more biomarkers are currently being investigated. For example, the unmethylated phosphodiesterases gene (U-PDE9A) may be a useful biomarker for the detection of fetal Down's syndrome using a blood test during the first trimester of pregnancy.51
Research in biomarkers related to cancer is growing. A study conducted in Colombia by Navas and colleagues found that out of 49 patients with liver cancer, 58% had biomarkers for hepatitis B, a risk factor for liver cancer, suggesting that both hepatitis B and its biomarkers could serve as possible biomarkers for liver cancer.52 Prostate cancer is one of the most common cancers affecting men and could be detected early using a biomarker like prostate-specific antigen (PSA). However, PSA testing cannot differentiate between benign and malignant pathology and has low sensitivity.
Recently, serum interleukin-18 was found to have higher sensitivity and specificity than PSA, indicating it is potentially a better diagnostic biomarker for prostate cancer.53 A recent Japanese study indicates that ATP-dependent RNA helicase DDX39 may be a novel prognostic biomarker for gastrointestinal stromal tumors.54 Protein-based biomarkers present in cyst fluid can aid in diagnosing the type and malignancy of cystic pancreatic tumors.55 High-sensitivity CRP has been identified as an independent biomarker in predicting risk of cardiovascular disease, such as major coronary events after coronary stenting. Information related to biomarkers is constantly improving as more research is conducted. Awareness of test outcomes and their potential impact on patient care enhances health care professionals' ability to make optimal care-related decisions.
The future of biomarkers. At a time when the focus of health care is growing increasingly personalized or tailored to the individual, biomarkers are becoming the tools of choice to select and guide therapy in clinical practice. Advances in proteomics and genomics will enable researchers to better understand diseases unique to a given patient population and, thereby, determine the optimal course of treatment. Moreover, biomarkers will be used to identify the groups of patients who will respond positively or negatively to specific treatments.
Advances in proteomics are enabling researchers to identify more specific biomarkers for various diseases.56 For example, by studying protein structure and function, researchers can identify noninvasive, blood-based diagnostic biomarkers to differentiate healthy control subjects from asthma patients. This will include analysis of blood proteins involved in the iron metabolism pathways. The specificity of this approach would be an alternative to the exhaled breath biomarkers like eNO.
Another example of recent advancements in biomarker research is a troponin test that allows clinicians to detect very low circulating levels of the protein. However, this presents challenges for clinicians because patients not suffering from AMI may have low detectable troponin levels as well.57 Some researchers suggest that troponin be sampled across patient populations in order to refine risk assessment and increase individualization.
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