By Dr Ritu Bisht
Platelets are the smallest cellular elements of the blood. They are anucleate fragments essential for clotting. Their inherent property to “stick” and form clumps and plugs helps to stop a bleeding. Paradoxically this property becomes a challenge, when labs need to count them as they have to artificially be made “non sticky” to enable a count.
It sure is vexing for the scientific community when results are compared between labs by non experts without the judicious use of fundamental tenets of laboratory principles.
Reading these newspaper reports prompts me to pen some facts for those interested in the technology that exists.
So let’s understand the science behind the Platelet Count test (Analytical phase).
- The Auto Analyser is the most accurate method of counting platelets. However, it has its limitations.
- The analysers used to measure platelet count are less accurate in measuring platelets <30,000/uL, which is the level at which hospitalisation is considered.
- Platelet count of <100,000/uL by the analyser must be crosschecked by the microscopic evaluation of blood smear to rule out platelet clumping.
- Microscopic evaluation of blood smear is a subjective analysis. It has high intra and inter-observer variation, which may go up to 40% in cases where platelet count is <30,000/UL.
- The allowed variation in Platelet Count testing as per the recommendations of NABL/CLIA is+/- 25%
- Accuracy of the test also depends upon the principle of counting used by Analyser. Some analysers use all 3 methods. Types of analysers are:
- Impedence: Count platelets depending upon size;
- Light Scattering: More accurate than impedence;
- Fluorescence: More accurate than impedence and light scattering;
- Immunological – is a reference method (Not used routinely).
- Many analyser related considerations are also important for the accuracy of the results.
- Is the analyser calibrated (standardised to give accurate results)?
- Is the calibrator traceable to ICSH (International Council for Standardisation in Haematology)?
Just as the jewellers ensure that the standardised weighing machine is used, similarly all analysers need to be calibrated as per ICSH at regular intervals.
- Is the adequate maintenance of analyser performed regularly as per the recommendations and validated for Linearity and Precision?
- Is daily background check of the analyser and reagents done daily before starting the testing?
- Is daily Quality Control done at 3 levels and Bias evaluated?
- Is Proficiency Testing conducted?
- Is QC monitored for Trueness, Accuracy, % CV & Bias?
- Is the Analyser performing as per specifications? SIGMA METRICS is used to measure the quality which is monitored and checked by accrediting agencies whose aim is to guide consumers to a lab you can trust.
Sensationalising mild differences in the platelet count which have no bearing to therapy or management, confuse, create doubt and hurt an already fragile patient doctor relationship.
The last bit of personal experience is that patients must be counselled if a result is worrying, unexpected or does not correlate clinically.The patient is “most helped” if they go back to the lab and let the pathologist recheck the sample along with a fresh sample rather than go on a crosscheck spree.
Responsible and restrained efforts are important by all in the healthcare services in the benefit of the common cause.
(Dr Ritu Bisht, MD, Pathology, has 18 years of experience in Laboratory Medicine and has headed and managed Quality for 15 years in NABL accredited labs.)
