Good-bye Agar-Agar: What's Coming for Infectious Disease (ID) Diagnostic Tests (Acute and Convalescent ID Doctors).

Abstract

Thirty years ago, many of us could not spell PCR (polymerase chain reaction). Common and uncommon diagnoses were often made by debate and inaccurate clinical suspicion. And if the patient was fortunate, our selected treatment was of broad enough spectrum to help an immune system effect a cure. Our intentions were no less valiant than those of any era; however, by today's standard, the laboratory support was then at the beginning stages. YESTERDAY is a quick look at the laboratory “back then”: agar-agar was 2 words, most pathogens could not be cultured, and pathogens that grew were often misidentified using visual inspection, smell, and a few chemical reagents. Complement was fixed when it was not broken, and infections were sometimes diagnosed by something called convalescent titers long after the diagnosis was clinically useful. In addition, if no diagnosis was evident by all available means, it was then classified as a “virus” … one of those poorly understood DNA-RNA things that can cause an illness for which there was no treatment. Therefore, testing was stopped. We also did not have financial pressures to stop ordering unnecessary tests. TODAY is our everyday practice. Because of the diligence and expertise of many, we have come a long way. We live it, we use it, and there is no need to review it here for this audience. TOMORROW we will begin to see some peeks into the immediate future. So let us talk about the laboratory support that is likely to become available to an infectious disease clinician; probably sooner than one might think. Ideally, a diagnostic test should be accurate, precise, quick, clinically useful, and employ a method (also called a platform) that can be easily performed in most patient care areas. To achieve these goals, we must look at alternatives to cultures of body fluids. Although cultures have served us well, every clinician can quickly list many limitations, and laboratorians know a dozen more. TOMORROW our alternative to cultures is to determine the presence of an infecting organism by identifying a tiny molecular piece of the pathogen (or even a unique metabolic product) that is so specific that the diagnosis is almost certain. Nucleic acid-based amplification technologies (NAAT) are already in use. Nucleic acid-based amplification technologies, using PCR to amplify a specific piece of a nucleic acid, have already increased the sensitivity and specificity of bacterial identification and simultaneously improved turnaround time. In addition, commercially available NAATs can identify sequences that underlie drug resistance and detect microorganisms the require special consideration for therapy and infection prevention and control. One limitation of most currently available molecular tests is that the clinician has to know what organism is being sought. For example, requesting herpes PCR tells you whether herpes is present but no additional information. Likewise, a rapid test for Group A streptococcus or methicillin-resistant Staphylococcus aureus identifies the presence of only what is requested. In contrast, a body fluid culture can recover hundreds of potential pathogens with just 1 set of media. Therefore, the burgeoning technology will be an attempt to merge these features by using panels of molecular reagents with tens, hundreds, or even thousands of different substrates. Imagine putting a drop of blood into a test system and learning in minutes that a patient is bacteremic for Klebsiella pneumoniae carbapenemase, Escherichia coli, and bacteroides. In addition, envision panel testing spinal fluid that would identify all enteroviruses, arboviruses, herpes simplex, and common bacterial pathogens with a turnaround time that is clinically meaningful. We already have a few commercial molecular diagnostic products that “multiplex” (identify more than 1 target), such as 1 test for gonorrhea and/or chlamydia and another single assay for a multitude of different respiratory viruses. Without a doubt, more tests are on the way. As a quick overview, a simplified glossary for the laboratory in our future might include the following.