In the laboratory we’re constantly seeking ways to check that the test results that we put out are accurate. Our primary reason for doing this is that we want to make sure the patient is treated appropriately based on the results of the tests we run. Also, it’s nice not to release values that appear to be nonsense. A tool that is sometimes used to help us check results is something called feasibility limits.
Lab computer systems often allow you to enter feasibility limits for your test results or for your test parameters. These are values outside of which you would not expect to find an analyte concentration in a living person. For example, you might expect a serum creatinine of 200 mg/dL (17,680 µmol/L) in a zombie, but you wouldn’t expect to find one in a living human being. Setting feasibility limits helps you catch things that make no sense before the physician calls you on them, for instance if you have a decimal malfunction and mistakenly try to report a plasma calcium value of 90 mg/dL (22.5 mmol/L) instead of 9 mg/dL (2.25 mmol/L). The trick to feasibility limits is deciding on the highest or lowest value you might expect to see in a living human being. In the case of calcium, upper feasibility limits of 20 mg/dL (5 mmol/L) may give you wiggle room without letting you report nonsense. However, feasibility limits have their drawbacks also. One of those drawbacks is that I’ve found that with human beings, nearly anything is feasible, especially in the pediatric realm. I finally removed the feasibility limits from the LIS in my institution, after a couple of different episodes led me to that conclusion.
One was a body temperature on a blood gas analysis. Under normal circumstances one would not expect to encounter a body temperature much below 90° F (32.2 °C) ever. But of course hospitals are not known for their populations being “under normal circumstances”. The body temperature of the patient in question was 70° F (21°C) on a patient who had been cooled down for surgery. The blood gas instrument accepted the temperature, but the lab computer system would not because of the feasibility limits set in the computer. The patient’s blood gas results could not be released in the computer until we took the temperature feasibility limits out.
Another example was with sodium. It would seem reasonable to have an upper feasibility limit of 180 mmol/L for sodium. Yet we had a patient whose sodium was 199 mmol/L when he arrived in the ED. Reasonably expecting some sort of contamination issue, we requested another sample, which had a sodium of 204 mmol/L, followed by 200 mmol/L in the next sample. These were real sodium results and over the course of several days’ time the physicians managed to get the patient’s electrolytes normalized. Again, feasibility limits interfered with result reporting and had to be removed from the computer.
These episodes caused us to remove most of our feasibility limits from the computer. They also helped me to remember and important point: Tools are fine, but you must understand their uses and their limitations in order to use them appropriately. Feasibility limits can be useful as long as you keep in mind that with humans, you often see the unfeasible.
-Patti Jones PhD, DABCC, FACB, is the Clinical Director of the Chemistry and Metabolic Disease Laboratories at Children’s Medical Center in Dallas, TX and a Professor of Pathology at University of Texas Southwestern Medical Center in Dallas.