Testing for Infectious Diseases: The Basics

09.30.2021

Contributors: Liangqi Ouyang, Meg Richardson

Overview

Ordinarily, infectious diseases account for only a small percentage of overall global deaths. However, if infectious disease cases are not properly monitored, they can become dangerous and can wreak havoc on health systems and economies. For this reason, disease detection is  extremely important.

How Infectious Diseases Spread

Infectious diseases are caused by microorganisms such as viruses and bacteria. They can spread in a number of ways including:

  • Person-to-person contact
  • Insect or animal-to-person contact
  • Contaminated surfaces
  • Waterborne or airborne particles
  • Exchanges of fluids

The Impact of Infectious Diseases

Some of the most common infectious diseases include influenza, HIV/AIDS, viral hepatitis, and tuberculosis. COVID-19 is also a deadly infectious disease. The COVID-19 pandemic alone has caused 185.05 deaths per 100,000 people in the US1 which marks a significant increase from normal years.

In 2020, COVID-19 was the third leading cause of death in the US, surpassing strokes, Alzheimer’s disease, and diabetes. The disease was largely responsible for the 17.7% overall increase in the number of deaths from 2019 to 2020 in the US.2 The COVID-19 pandemic has highlighted just how dire infectious disease outbreaks can be, and why it is so important to monitor them with testing.

Detecting Infectious Diseases

To detect an infectious disease, a test must identify a bacterium, virus, or other microorganism in a patient.5 Laboratory tests can identify these microorganisms by analyzing a sample from a patient such as fluid or tissue. Some of the most common types of tests include microscopy tests, culture tests, immunologic tests, nucleic acid-based tests, and non-nucleic acid-based tests.6

Below is a description of each type of test:

  • Microscopy tests are a rapid and inexpensive way to detect certain infectious diseases in specimens.7,8 Accurate interpretation of a sample from this type of test requires some training and experience.7 There are several different types of microscopy tests which serve different purposes. For example, electron microscopic diagnosis is uniquely suited for identifying infectious agents quickly. With this method, an experienced virologist or technologist can identify a pathogen in about 10 minutes.9,10
  • Culture tests are a detection technique where microorganisms are grown in a growth medium. Culture can make certain microbes easier to identify and detect. The time needed for culturing depends on the types of microorganisms and the type of growth medium. However, a test using culture usually takes longer than a microscopy test.
  • Immunologic tests are used to detect infectious diseases by identifying antigens from the infectious agents or antibodies from immune responses. They deliver results in a short period of time.13 These tests are commonly used in not only infectious disease detection, but also for pregnancy tests, allergy tests, and drug tests.
  • Nucleic acid-based identification methods detect pathogens which cause infectious diseases by identifying pathogens’ specific genomic sequences (DNA or RNA). These tests provide fast, accurate, and sensitive results.14 This method usually involves two steps. The first step is pre-treating the sample, for example, extracting and purifying target nucleic acids. The next step is genetic analysis, which often includes a nucleic acid amplification step.14 Reverse-transcription polymerase chain reaction (PCR) is one of the most commonly used techniques for amplifying a target DNA fragment. Compared to antibody-based detection, PCR tests are more accurate. They can also detect viruses early in the infection cycle, and exhibit higher sensitivity which results in fewer false negatives.14
  • Non-nucleic acid-based identification methods are based on functional or morphologic characteristics of organisms instead of their genetic information.15 These tests can identify a microorganism by detecting specific proteins which it contains or produces.16 For instance, mass spectrometry can detect various proteins of different masses in a specimen. The relative mass and abundance of unique proteins in specific pathogens can sometimes be used to identify them.15