Hematology analyzers can do a lot of things these days. They can do a complete blood count, or tot up red or white blood cells, or platelets. They can tell you hemoglobin concentration, hematocrit, RBC indices, and leukocyte differential. Even older machines can do a limited CBC, and a three-part leukocyte differential, giving values for neutrophils, lymphocytes, and all other white cells together. The snazzier ones count monocytes, basophils, and eosinophils too. They can also detect nucleated red blood cells.
In certain situations, hematology analyzers are known to provide a falsely low platelet count. This worries some of our customers. In this blog post we’ll talk about that, and explain why not every machine is the same.
The accuracy of platelet count is undeniably critical sometimes. It’s how you assess the risk of bleeding in patients, for one thing. Decreased platelet count is seen in things like thrombocytopenic purpura, aplastic anemia, and leukemia. Too many platelets can mean primary thrombocythemia, polycythemia vera, or chronic myelogenous leukemia. You want to get the numbers right for diagnosis and differential in clinical hemostasis and thrombotic diseases.
Over the years, platelet counting has been done by microscopic tallying, automated blood cell analyzers, and the “international reference method” (or “IRM”), based on flow cytometry, as proposed by the International Council for Standardization in Haematology (ICSH). There’s an “impedance” method in wide use, invented by Beckman Coulter. It has its limitations, like being unable to distinguish platelets from particles of similar size and volume. Instrument manufacturers have tried hard to work around this. The Advia 120, Cell-Dyn 4000, and Mindray BC-6800 are optical platelet counters that differentiate blood components by type and strength of light signals. Another analyzer, the Sysmex XN-3000, does it entirely differently, with fluorescent nucleic acid staining.
Head-to-head studies are being done all the time these days. One big one recently evaluated several iterations of optical fluorescence counting machines in light of thrombocytopenia and thrombocytosis. Investigators found that the correlation coefficient between the Sysmex XN-9000-F (r = 0.988) and their gold standard, the IRM, was larger than that of the Sysmex XN-9000-O (r = 0.984) or the Sysmex XN-9000-I (r = 0.937) for thrombocytopenic samples. Compared with the IRM, the average bias of the Sysmex XN-9000-O and Sysmex XN-9000-F was the same, but the 95% LA of Sysmex XN-9000-O (95% LA = -10.8 to +7.8) was wider than the Sysmex XN-9000-F (95% LA = -9.4 to +6.4) for thrombocytopenic samples. Interesting, that they’re not all the same, even within one counting modality.
Across modalities it gets even more interesting. One separate study has found that the fluorescence method is more suitable for platelet count in patients with thrombocytopenia than the optical and impedance methods.
Another study, however, appears to show that the impedance method is actually superior to optical and fluorescence methods in thrombocytosis patients. The differences between the two studies’ results may be due to differences in study design. One of the studies used samples from thalassemia patients, while the other did not restrict to a specific disease population. Both studies, incidentally, found that optical methods were more consistent with the IRM than the impedance methods for counting platelets at normal levels. This was true in the Sysmex XN-9000 instrument and also the BC-6000 Plus instrument.
The impedance method looks at each platelet passing through a small hole. This detects only volume. So it gets confused with big platelets, small red blood cells, platelet aggregation, and cell debris. Fancy machines like the BC-6000 Plus and the XN-9000 get around this with nucleic acid fluorescence staining. This shows up RNA and part of the DNA in cells, and finally distinguishes different matter by the intensity of light signals. There are external confounders sometimes too, like interfering substances in peripheral blood cell smears.
So why doesn’t everybody use the fluorescence method for every sample? Because it’s expensive, that’s why. This is what a very recent study ended up saying, having also minutely considered the different techniques in particular clinical syndromes. This study looked at 23 different automated blood analyzers, found that most of them produced higher results than the IRM, but all had acceptable sensitivities and concordance rates with the reference method for diagnosis of thrombocytopenia and thrombocytosis. As such, the investigators judged that all 3 automated methods can be used for the screening and ruling out of thrombocytopenia and thrombocytosis in patients with thalassemia. However, they did observe that the specificity of the PLT-I was low for the diagnosis of thrombocytopenia. They ruled that an additional assay test should be used to improve the specificity of thrombocytopenia diagnosis when the PLT-I value is lower than 1003103/lL. PLT-O or PLT-F can be used in this setting owing to their higher specificities and concordance rates with the IRM. That’s if you can afford it. What you buy for your own lab or clinic will probably depend on what you’re trying to do.
If you’re shopping with us, you might have a look at the Beckman Coulter AcT 5 diff AL analyzer. It does away with manual sample loading, which keeps things quick and cheap. It’s also got a very streamlined data management system that takes the headache out of data recall. There’s also the Mindray BC-2800 Auto analyser. This one is fully automated, and, for what you get, very low-cost. It manages 3-part WBC differentiation, with 19 parameters and 3 histograms. It’ll do 30 samples an hour, and you can store results for 10,000 of them (and that includes histograms).