Comparison of White Blood Cell and Platelet Counts Between Ficoll-Paque and Standard In-111 WBC Labeling Techniques

Objectives: In-111 WBC scintigraphy is a useful tool in patients with known or suspected infection, and to maximize specificity, it is important to minimize labeling of red blood cells and platelets. The standard In-111WBC label is utilized in cases of suspected osteomyelitis, soft tissue infection, and fever of unknown origin (FUO). Our institution utilizes the Ficoll-Paque method for cases of possible vascular infection (i.e. arterial endografts, bypass grafts, arterial aneurysms, etc). Because platelets may adhere to thrombus along the walls of arterial grafts and aneurysms, limiting inadvertent In-111 platelet labeling is desirable to prevent the possibility of a false positive examination. As Ficoll-Paque is a density medium that separates whole blood into distinct layers after centrifugation, it should theoretically decrease platelet labeling and therefore be preferable in cases of vascular infection. However, it requires extra time and manipulation of the sample which may be unnecessary. Therefore, the aim of this study was to compare the standard In-111 WBC labeling protocol to the Ficoll-Paque density media protocol by quantifying the numbers of labeled platelets and WBCs in the final cell pellet.

Methods: This prospective study was performed on patients who underwent In-111 WBC scintigraphy at Mayo Clinic from 2015-2017. Twenty-two patients had blood drawn for In-111 WBC scintigraphy. A small sample (100uL) of whole blood drawn for the procedure (approximately 60 mL) was evaluated with a hematology analyzer and initial values of WBCs and platelets were noted. The whole blood sample then underwent the ordered preparation (Ficoll vs. Standard) and was processed down to the final WBC pellet. The processed WBC pellet was then analyzed using the hematology analyzer, noting platelet and WBC counts before being labeled with In-111 oxine. Differences in WBC and platelet counts between the two methods’ final processed cell pellet were noted, as well as the percentage of platelets that remain in the final cell preparation.

Results: The average final WBC concentration using the Standard WBC labeling method (84.8 ± 40.9 x10³/ul) and Ficoll method (80.2 ± 76.2 x10³/ul) were not statistically different (ANOVA p = 0.43). The Ficoll method was superior in platelet removal. The average number of platelets remaining in the final cell pellet using the Ficoll method was 24.3 ± 26.8 x10³/ul, significantly lower than the Standard method which averaged 49.0 ± 37.0 x10³/ul (ANOVA p = 0.04). The composition of the final processed cell pellet was also assessed between Ficoll and Standard methods. The Ficoll method averaged 27.2% platelets (72.8% WBCs), while the Standard method averaged 33.7% platelets (66.3% WBCs).

Conclusion: In most situations, the standard WBC labeling method will suffice as it gives a high WBC concentration while containing on average 33.7% platelets. However, the platelet reduction ability of Ficoll-Paque remains a useful method for imaging vascular infection, where radiolabeled platelets may create a false positive test result.