Giant platelets.

Q We have a patient with giant platelets. He carries two diagnoses (1) lymphoma, which was treated with a splenectomy, and (2) essential thrombocytosis, which is being treated with hydroxyurea. It had been the practice in the laboratory to dilute the patient's blood sample 1:5 and then count the diluted specimen on an automated hematology analyzer; the results were then multiplied by 5. It was my view that giant platelets, like nucleated RBCs, will artifactually elevate the WBC when counted on a hematology analyzer. In addition, the giant platelets are not counted as "platelets" by these instruments.

Because our laboratory does not perform manual/visual blood counts, it has been the practice to send these specimens to a referral laboratory to ensure accuracy; however, the referral laboratory does not perform manual counts if the blood film review appears to agree with the instrumental count. Another recent complication has been platelet clumping when there is a delay before counting. Needless to say, the patient's physician is quite concerned because he needs accurate platelet counts to adjust the dose of chemotherapy.

My solution to this problem has been to promptly count the specimens in our laboratory (to avoid platelet clumping) and to do a giant platelet/WBC differential on a well-prepared blood film. This is similar to the method used to correct the WBC count when nucleated RECs are present. Using this method, it was noted that there were 135 giant platelets for every 100 WBCs on the blood film, meaning only 42% of the "WBCs" counted by the instrument were authentic WBCs. The automated "WBC" of 5.7 X [10.sup.9]/L, therefore, was corrected to 2.4 X [10.sup.9]/L. We calculated the corrected platelet count to be 464 X [10.sup.9]/L when the instrument counted 313 X [10.sup.9]/L platelets. Are these methods valid?

A Giant platelets may be found in some congenital conditions, such as Bernard-Soulier syndrome, and in some myeloproliferative and lymphoproliferative disorders. Furthermore, they may become more prominent in patients who have undergone a splenectomy because the spleen does the work of removing abnormal cells from the circulation.

You have quite properly improved your laboratory service. Even though these studies are tedious, you took the appropriate course of action toward ensuring that your laboratory provides accurate and prompt data.

Our increasing reliance on automated hematology instruments has resulted in the occasional misclassification of giant (or dumped) platelets. The instrument manufacturers have done a reasonable job of programming their instruments to flag suspicious specimens so that blood films may be carefully reviewed and appropriate corrections made. Nevertheless, physicians must be informed of problems because such discrepancies can have a significant effect on the diagnosis and treatment of their patients.

The various hematology analyzers flag these problems in different ways. Therefore, laboratorians must be acquainted with the techniques used for counting blood samples and with the interpretation of histogram displays of the results. Finally, we continue to rely on blood film morphology to accurately identify these abnormalities.

But I think you may have overcorrected the platelet count. There are relatively few giant platelets. When you corrected the instrument "WBC" count of 5.7 X [10.sup.9]/L, you determined that 3.3 X [10.sup.9]/L of these cells were actually giant platelets. This number should have been added to the 313 X [10.sup.9]/L for a total platelet count of 316.3 X [10.sup.9]/L, not 464 X [10.sup.9]/L, as you reported. So the referral laboratory check of the blood film for the estimated platelet counts is reasonable (ie, the instrumental platelet counts are in order).

But it would be even better to report the giant platelets as a separate category because their function is most likely impaired. The number of normal-sized platelets more likely reflects the patient's true hemostatic status.

John A. Koepke

Daniel M. Baer is professor emeritus of laboratory medicine at Oregon Health Sciences University in Portland, OR, and a member of MLO's editorial advisory board.