NP Insights By Tom Bartol, APRN

Our current health- care culture empha- sizes evidence-based treatment. Diagnostic testing should also be

evidence-based. Tests are sometimes ordered without considering the evidence behind them. Clinicians may order a diagnostic test out of fear or to offer reassurance to the patient. Ineffi cient testing can lead to increased costs as well as unneces- sary or unwanted treatment for some patients. Using evidence to guide diagnostic testing can become part of the shared decision-making process, giving the patients a perspective about what the test might mean for them. The patient and clinician can then make a choice that fi ts with the patient’s condition as well as the patient’s desired goals and values.

This process need not add immense complexity to the decision- making process. Four steps can make the process more thoughtful and effi cient. First, determine the pretest probability of the condition you are concerned about. If you have no idea what you are looking for or have no differential diagnoses, then a test is probably not the way to begin. Second, determine what you want from the test. Do you want to rule out or rule in a disease or condition? Next, understand the sensitivity and specifi city of the test you want to use. Finally, think about what you will do with the results of the test.

■ Pre-test probability Pretest probability is the likelihood that a patient has the condition you are considering prior to testing. This can be based on the prevalence of the condition in the population. For example, the prevalence of colon cancer in the average 50-year-old female patient is about 0.1% or 1 in 1,000.1 If that female had a family history of colon cancer, heavy alcohol use, little physical activity, or other factors that increase risk for colon cancer, the pretest probability would be higher. Frequent exercise or a high-fi ber diet would lower pretest risk. Pretest probability can vary based on symptoms or clinical conditions as well. Consider the case of a 59-year-old male presenting with left-sided chest pressure. The pretest probability would be lower if the pain is sharp and aggravated with deep breathing and higher if the pain is worse with exertion, accompanied by shortness of breath, nausea, and diaphoresis. A past history of coronary artery disease (CAD) or a history of hypertension and diabetes in this patient would also increase pretest probability.

Determining pretest probability can sometimes be challenging. For various types of cancer, the pretest probability or incidence can be found on the CDC website (cdc.gov). In many cases, you will not be able to fi nd an exact percent or number for the pretest probability. Simply determining if the probability is low, medium, or high can be very helpful in making testing decisions. For example, consider pretest probability

for diabetes with two different people. The fi rst is a thin, 65-year- old male with no family history of diabetes, normal BP, and lipids who would have a low pretest probability. The second is a 58-year-old obese male with hypertension, hyperlipid- emia, and two brothers with diabe- tes; this patient would have a high pretest probability. A general sense of pretest probability for many conditions can be determined through the history and physical exam of your patient.

■ Testing goals Next, consider what your goal is for the test. Do you want to rule in a diagnosis or rule out a diagnosis? For those with a high pretest probability of a condition, you will likely be ruling in a diagnosis, and for those with low pretest probability, ruling out will be the goal of the diagnostic test.

By using pretest probability and understanding what you want to do with a test, you can compare the sensitivity and specifi city of a test to help determine how each test will help you with your goal. Understand- ing sensitivity and specifi city can be challenging for some clinicians. An easy way to remember is that a highly- sensitive test that is negative rules out a condition, whereas a highly-specifi c test that is positive rules in the condition. To help remember this, think “SNOut” for sensitivity negative rules out and “SPIn” for specifi city positive rules in.

Finally, before ordering a diagnostic test, consider the implica- tions of the results, that is, what you

Thoughtful use of diagnostic testing: Making practical sense of sensitivity, specifi city, and predictive value

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NP Insights

www.tnpj.com The Nurse Practitioner • August 2015 11