Boulder in the way of the coulter – Interferences in CBC analyzers

Foreword : Lab medicine employs a fascinating array of technologies that can be poles apart from each other. The entire repertoire may range from the humble card test which might actually be translated to the complex term ‘immunochromatography ‘ to state of the art technology that can actually quantify matter. When such refined technology is put to use there is always scope for the rare glitch to creep in. Interference is lingo that lab medicine practitioners use for factors that can cause quirks in lab reports. Scientifically put, interference is either an endogenous or exogenous substance which falsely alters the true value & thereby the test result. This may lead to inappropriate further tests, incorrect diagnoses, and treatment with potentially unfavourable outcomes for the patient. The most frequently performed interference studies are for variables in biochemistry such as hemolysis, icterus & lipemia.The following lines attempt to shed light on interferences which can alter blood count values.

Interferences in hematology cell counters

While there is overwhelming literature on interferences in immunoassay & biochemistry, the available scientific content available on interferences in hematology is slender when compared with the former. Few practical examples of interferences specific to hematology coulters are listed below -

EDTA :

• The morphological changes that EDTA induces in blood samples has been studied comprehensively.
• For instance tripotassium EDTA being a liquid dilutes the blood sample & influences RBC size significantly when compared to its di-potassium counterpart.
• EDTA induced platelet satellitism is another phenomenon which is observed with prolonged contact time. Inadequate vacutainer inversions & long contact time with EDTA,more than what is recommended can contribute to false blood counts. Especially altering the platelet count. The frequency of EDTA induced pseudo thrombocytopenia ranges from 0.07 to 0.11 of all blood counts.

Significant changes in cell morphology appear when the contact time with EDTA exceeds 24 hours. Robust phlebotomy practices & use of standardized equipment for blood collection will augur well for testing labs.

Lipemia :

• Lipids can interfere with light scatter & potentially influence the assayed values.
• Some of the methods employed by laboratories for clearing up lipemic samples & making them acceptable for testing include the saline replacement technique & extraction of lipids using polar solvents such as polyethylene glycol or cyclodextrin. Such agents are commercially available & can be out to use.

Microclots :

• Most hematology coulters are equipped with technology to pick up suspect flags such as abnormal cells,platelet clumps,debris?& clots.

However it is important for laboratories to diligently ?review flagged samples manually for microclots.The most common 'analyte' affected as a result of microclots being present in the sample is the platelet count.Pseudothrombocytopenia could be potentially reported & treated if the technical team overlooks microclots in the sample.

Many a time the robustness of the slide review criteria determines the quality of the CBC. This reiterates the importance of competency assessment of technologist’s involved in the screening of peripheral smears.

Other interferences : ?Cryoglobulins,nucleated RBC’s,Red cell inclusions,RBC fragments & drugs can also alter the values given by the coulter.

Afterword : The coulter technology has received fine tuning over the years. Many manufacturers of hematology analysers have updated their arsenal & have come up with technical advances that would delight the seasoned physicist. However adherence to the basic principles of lab medicine ‘waste in,waste out’ is integral to practice & is of paramount importance.