Who makes doctors look smart?

When your doctor takes a blood sample or a throat swap, then gives you the results some time later, you probably think little about who actually analyzes those samples. But without these people—clinical lab technicians—working inconspicuously, doctors would not be able to determine your blood sugar, test you for strep throat, or do any other kind of chemical test.

I toured the clinical lab at the University Health Services and observed the techniques that the clinicians use to analyze samples of all sorts. Say that you come into UHS with a sore throat. A lab technician will take your throat swap and spread it onto a Petri dish full of a type of food called MacConkey agar, then incubate the dish for several days. Only gram-negative bacteria can grow on MacConkey agar; a common cause of strep throat, Streptococcus pyogenes, is gram-positive, so growth on MacConkey agar rules out this bacterium as the infectious agent. If there is no growth, the technician will culture the bacteria on another plate containing blood agar, which is opaque red; if the bacteria make the agar transparent, they are likely S. pyogenes.

Streptococcal_hemolysis

Streptococcus growing on blood agar in section marked “β.” Y Tambe. Wikimedia Commons.

If the infection is less clear, the technicians will culture the bacteria in a plastic plate that contains over 100 wells, each filled with a different chemical, such as glucose or lactose, and a dye. As the bacteria grow, they consume specific chemicals, causing certain dyes to change color; each species produces a characteristic pattern of color changes that the technician can use to identify it. For example, most Escherichia coli strains ferment glucose, but not sucrose, producing acids that cause the dye to change color in only the glucose well, while most Pantoea ananatis strains ferment both sugars, changing the color of both wells. This way, technicians can distinguish between bacterial species.

To diagnose other conditions, infectious and not, technicians will examine almost any body fluid microscopically. Say that you are having blood work done. The clinicians will take a small amount of blood and feather it out onto a glass microscope slide. I was allowed to inspect a sample at 1000x magnification, at which point red blood cells were visible as indigo ovals with pale centers and white blood cells were purple, more irregular blobs. The white blood cells were pressing up against the red blood cells, which is indicative of mononucleosis. I also was able to inspect a urine sample that a technician had placed into the narrow gap between two plastic plates, much like in a hemocytometer. I saw tiny X-shaped crystals of calcium oxalate, which can constitute urinary or kidney stones, and thus its appearance in the urine is a warning sign.

Infectious mono

White blood cells (purple) pushing against red blood cells (pink), indicative of mononucleosis.

The heart of the diagnostics, however, is the array of test strips. To accurately test for mono, for example, the technicians take a piece of plastic that encloses a strip of paper treated with chemicals that change color upon exposure to molecules in the blood. They put a drop of blood through a hole in the plastic and onto the test strip; as the blood diffuses across the strip, it causes one or two spots to turn purple. One spot is the positive control: it should turn purple whether or not the patient has mono, and if it stays white, the test result is void. Assuming the control works, a color change in the other spot indicates mono. Technicians test for many conditions, such as pregnancy, with test strips like these.

Test Strip

A cartoon test strip, showing positive, negative, and invalid results.

When the change in color is not “Yes or no?” but rather, “How much?” technicians place the treated test strip into a machine called a colorimeter that accurately measures the color. (In the past, technicians had to estimate the colors with their own eyes.) For example, blood glucose test strips have a red circle that changes color when exposed to glucose; the more glucose, the more the color changes. A colorimeter about a meter cubed reads the test strips for glucose and triglycerides. I realized that many times in my nutrition blog and issue brief, I had talked about blood glucose and triglycerides, but never had I wondered exactly how they were measured. But seeing the colorimeter that does the job was fulfilling nonetheless.

What struck me most about the clinical lab was its reliance upon prefabricated chemical tests. Test strips, colorimeters, bacterial well plates—they all come ready to use, and this gave me new respect for the industry that mass-produces these diagnostic tools and makes them ever more efficient. The technicians told me that there are fewer schools that offer programs in clinical lab work now than there were twenty years ago, and that few people are entering this field; consequently, there will be a job shortage in ten years if trends continue. Though job prospects are promising and the median salary is about $57,000, I do not think that I would like to be a clinical lab technician. I recognize their importance in diagnostics, but I would like to go into a research career focused more on developing new technology or knowledge. But, as always, ruling against a potential career still helps me to decide what I would likely rather do.

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