Accuracy of Platelet Counting by Optical and Impedance Methods in Patients with Thrombocytopaenia and Microcytosis

Abstract

Objectives: Obtaining accurate platelet counts in microcytic blood samples is challenging, even with the most reliable automated haematology analysers. The CELL-DYN(™) Sapphire (Abbott Laboratories, Chicago, Illinois, USA) analyser uses both optical density and electronic impedance methods for platelet counting. This study aimed to evaluate the accuracy of optical density and electrical impedance methods in determining true platelet counts in thrombocytopaenic samples with microcytosis as defined by low mean corpuscular volume (MCV) of red blood cells. Additionally, the impact of microcytosis on platelet count accuracy was evaluated.

Methods: This study was carried out between February and December 2014 at the Haematology Laboratory of the Sultan Qaboos University Hospital in Muscat, Oman. Blood samples were collected and analysed from 189 patients with thrombocytopaenia and MCV values of <76 femtolitres. Platelet counts were tested using both optical and impedance methods. Stained peripheral blood films for each sample were then reviewed as a reference method to confirm platelet counts.

Results: The platelet counts estimated by the impedance method were on average 30% higher than those estimated by the optical method (P <0.001). The estimated intraclass correlation coefficient was 0.52 (95% confidence interval: 0.41-0.62), indicating moderate reliability between the methods. The degree of agreement between methods ranged from -85.5 to 24.3 with an estimated bias of -30, suggesting that these methods generate different platelet results.

Conclusion: The impedance method significantly overestimated platelet counts in microcytic and thrombocytopaenic blood samples. Further attention is therefore needed to improve the accuracy of platelet counts, particularly for patients with conditions associated with microcytosis.

Keywords: Anemia; Electrical Impedance; Mean Corpuscular Volume; Optical Devices; Platelet Counts; Thrombocytopenia.

Figure 1A–C:

Distribution of mean corpuscular volume (MCV) of red blood cells and platelet counts in thrombocytopaenic blood specimens with microcytosis (N = 189). A: Frequency of MCV distribution in the study population. B: Comparison of platelet counts among the optical method, impedance method and microscopic examination of blood smears. The minimum, maximum, median and P values for each boxplot are shown. C: Comparisons of platelet counts between the optical and impedance methods according to MCV values. The minimum, maximum, median and P values for each boxplot are shown. The open and filled boxes represent the optical and impedance methods, respectively.

Figure 2A–C:

Linear regression analyses of platelet counts between optical and impedance methods in thrombocytopaenic blood specimens with microcytosis (N = 189). The dotted lines show the 95% confidence band of the best-fit line. The correlation coefficient and P values are shown. A: Correlation analysis of platelet counts using the entire data set. B: Correlation analysis of platelet counts using samples with mean corpuscular volume (MCV) values of 60–70 femtolitres (fL). C: Correlation analysis of platelet counts using samples with MCV values of 71–76 fL.

Figure 3:

Bland-Altman plot depicting the degree of agreement between optical and impedance methods in platelet counts among thrombocytopaenic blood specimens with microcytosis (N = 189). The solid line represents zero difference between the two methods. The dotted lines represent the upper and lower limits of agreement between the two methods as measured by the mean ± two standard deviations.