I’ve heard it said that there is no such thing as a lab test with no interferences, and I have to admit, I believe that to be true. For every method devised to measure a specific analyte, something else can interfere with that measurement. For example, photometric measurements using absorbance assume that only the analyte of interest absorbs light at the wavelength being used. Quite often, many other compounds absorb light at that wavelength as well. In chromatography methods, we assume only the compound of interest elutes from the column at a specific time point, and again, many other compounds often do. Various types of mass spectrometry are touted as specific for the compounds being measured, however, even using mass spectrometry, compounds may fragment in similar patterns when looking at mass spectra, or fragment into the same size precursor and/or product fragments using tandem MS.
Thus, we routinely report test results knowing that most often what we are reporting is accurate. However, we must always be aware that the result we’re reporting may not be accurate due to interferences.
I recently had an occurrence related to test interference. Like all such cases, the tech responding to the clinician’s call used our standard response. He located the original sample and repeated the test. The assay gave the same results on the repeat and the result was reported back to the clinician as real and accurate, even when questions were raised by the healthcare staff about the result not fitting the clinical picture. And in fact, although the result was reproducible and in the realm of possibility, in this case the result was wrong.
The analyte in this case was plasma free hemoglobin which is performed in our lab by an assay which measures absorbance at one of the wavelengths at which hemoglobin absorbs light and subtracts a background wavelength reading. The test was persistently giving very high plasma free hemoglobin results even though the patient had no other evidence of hemolysis. When the healthcare staff became adamant about the discrepancy, the sample was sent to an outside lab which performs the assay using a full spectrophotometer, and the sample was found to have no hemoglobin present. An interferent in this patient’s sample was being measured as hemoglobin by our method.
Of course, once it’s been determined that a test is experiencing interference the next question from the healthcare provider is always, what is interfering? That’s a much more difficult question to answer, although occasionally it can be answered with some investigation. Looking into the patient’s drug regimen can help, as well as checking other health parameters to see what else is occurring. In the case of the elevated plasma free hemoglobin, the patient did have an elevated myoglobin which may have interfered.
The take home message here is that no matter how reproducible the results are, interferences are possible. As laboratory professionals, we should always be ready to look for ways to prove our results other than by repeating them, especially when the result does not fit the clinical picture and is being questioned by our healthcare colleagues. Sending the test to be run by a different method is one good way of determining interference. Another way is to check the patient’s chart for drugs or other substances that are known to interfere and are listed in the package insert. Finally, understanding the realities of assay interferences, and being willing to continue looking for answers is also important in the laboratory.
-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.