A Review of Epidemiology for the Physician Assistant Exam

When you take the Physician Assistant Exam (PANCE), your task is to recognize common terms used in epidemiology and to interpret some simple population-based data. Epidemiology is the branch of medicine that helps people figure out patterns concerning disease and other health-related conditions as they affect the population at large. For example, epidemiologists work to find the cause or source of epidemics of infectious disease.

In the Bad Old Days, tracing the origin and dissemination of a disease like bubonic plague was relatively easy. Travel routes were limited to sailing ships and poor roads. In the modern world, people travel a lot and move fast. The important work of the epidemiologist has therefore become more difficult.

How to identify conditions with lab tests

Specificity and sensitivity are words you see in the epidemiologic world big time. They both relate to the reliability of a laboratory test for a given medical condition.

Before reviewing specificity and sensitivity, you need to understand two other concepts: positivity and negativity. A positive test reveals the presence of the condition or disease for which the test is being done.

Say you’re dealing with someone who comes to your office presenting with a fever and productive cough. You order a chest radiograph, which shows a lobar infiltrate in the right lower lobe. His imaging study shows that pneumonia is present. This is a true positive test because not only does he have symptoms of pneumonia, but his chest radiograph shows that as well.

As you would expect, a negative test result doesn’t show the specific condition for which the test is being done. Test results may be false. For example, you’re evaluating a person with diabetes who feels “funny” and is sweating and tachycardic. You suspect hypoglycemia, but the person’s glucometer reports a normal reading. You send a blood sample to the lab, and it shows a blood glucose level of 30.

The glucometer reported that the blood glucose level was normal when in fact it was low. If the glucometer reading and the lab results were similar, this would be a true negative. However, because the glucometer differed from the blood test, the glucometer gave a false negative result. The person needs a new glucometer.

Concerning false positive testing, say you’re called about a lab value for a potassium level that is 8.0 mEq/L. If the potassium level were really this high, you’d expect to see changes in the vital signs, including bradycardia and an abnormal heart rhythm.

You would also expect to see ECG changes. You rush to see the patient, and the vital signs are normal, as is the ECG. You call the lab and find out this was a hemolyzed specimen. This is a false positive result.

In the medical profession, no diagnostic test is 100 percent foolproof. The ideal test would be both specific and sensitive.

  • Specificity deals with how reliable the test is in detecting a certain medical condition while minimizing the likelihood of false positive results.

  • Sensitivity deals with the probability that a test says a person has a certain medical condition.

Vectors in epidemiology

In epidemiology, a vector is any person, animal, or microorganism that carries and transmits an infectious agent — and there are plenty of vectors. Different vectors contribute to human disease, animal disease, and plant disease. Here are some animal stars of the vector world:

  • The Anopheles mosquito is responsible for transmitting human malaria. It’s the king of vectors. However, a new vaccine looks promising.

  • The Aedes mosquito can spread dengue fever and yellow fever.

  • Fleas can spread many diseases, of which bubonic plague is the most famous. Some estimates say that the plague has killed more than 200 million people.

  • In the United States, ticks of the genus Ixodes get special attention because they pass on the bacterium for Lyme disease.

  • In Africa, the tsetse fly is a scourge. It’s the vector of human African trypanosomiasis (sleeping sickness).

  • Here, kitty, kitty! House cats are the vector for Toxoplasma gondii, which causes toxoplasmosis, and for Bartonella henselae, which causes cat scratch fever.

The concept of vectors extends reasonably to two other areas: people and water.

  • Remember Mary Mallon, also known as Typhoid Mary? She was the first person in the United States identified as an asymptomatic carrier of the typhoid fever pathogen (Salmonella enterica, serovar Typhi). Her score: 53 people infected and 3 people dead.

  • If you lived near Broad Street in Soho, London, in 1854, dying of typhoid fever was easy. Contaminated water was the delivery vehicle. The physician who figured this out noted occurrences of the disease on a map that showed water wells. The doctor was John Snow, considered to be one of the fathers of epidemiology.

For an example, consider the Lyme test. The confirmatory test to truly diagnose Lyme disease is a Western blot. If a Western blot demonstrates that Lyme is present, then Lyme disease is highly likely to be present. This is a highly specific test. The number of people testing falsely positive for Lyme disease with the Western blot is low.

Incidence and prevalence of a condition

A common test question involves understanding the difference between incidence and prevalence:

  • Incidence: For any health-related condition or illness, incidence refers to the number of people who’ve newly acquired this condition.

  • Prevalence: Prevalence concerns the number of people who have this condition over a defined time interval.

Remember, because these are epidemiologic terms, they’re dealing with the population at large.

For example, say you wanted to investigate the number of cases of tuberculosis (and who wouldn’t want to do that?) in all people over the age of 65. Incidence is the number of new cases diagnosed in the past year, and prevalence is the number of people diagnosed who were over the age of 65.

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