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The primary care physician’s (PCP’s) decision framework and toolkit have evolved over the past two centuries. Diagnosis in the 19th century was driven by a patient’s history and physical examination. External cues interpreted by the physician’s senses served as input—the data was what the clinician could hear from the patient’s story, visualize, touch, smell, or even taste as was the case with tasting urine to diagnose diabetes.

The 20th century marked the addition of advanced diagnostic modalities such as laboratory testing, tissue analysis, and imaging that allowed physicians to interpret the body’s internal cues invisible to the naked eye. At times, this new medical technology led to the extinction of various parts of the history and physical examination, but more often it served as adjunct information.

The 21st century will be remembered as a time when genomic information became a key driver of clinical decisions. Although genomic information is also an internal cue invisible to the naked eye, it differs from traditional laboratory values and imaging. Genomic information is a permanent fingerprint that provides clues to historic and future states and insight into the pathophysiology of why the body reached a particular state, whereas laboratory values and imaging are snapshots of a patient’s state at a moment in time. Our hope is that use of genomic information might improve our ability to screen for disease, make accurate diagnoses, and allow for more appropriate therapies.

In this era of genomic-informed medicine, we will learn about new categories of diseases and mechanisms of disease that we did not previously know existed. Through genome sequencing technology, we can pool large populations of patients with similar symptoms and find genetic patterns, enabling us to discover novel disease-causing alleles and providing explanations for previously unexplained conditions. The genomic revolution promises to improve quality of care for patients and decrease cost for the healthcare system by driving more appropriate utilization of resources and decreasing adverse drug events. Adverse drug events and medication errors result in 700,000 emergency department visits, 120,000 hospitalizations, and $3.5 billion in medical costs. Even a modest decrease in adverse events would result in substantial savings.

The remainder of this chapter will help clinicians understand how we might apply genomic information in our clinical practices.

The Basics—Speaking the Language

Before we can discuss the clinical applications of genomics, let us review a few basic terms and concepts:

  • An organism’s complete set of DNA is called its genome. Virtually every single cell in the body contains a complete copy of the approximately 3 billion DNA base pairs that make up the human genome.

  • With its four-letter language, DNA contains the information needed to build the entire human body. A gene traditionally refers to the unit of DNA that carries the instructions for making a specific protein or set of proteins. Each of the estimated ...

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