mean cell volume (mcv), and calculate the hematocrit by multiplying the red blood cell count by the mcv. some measure the hematocrit by comparing the total volume of red blood cells to the volume of blood sampled, and derive the mcv from the hematocrit and red blood cell count. the hemoglobin concentration, the red blood cell count and the hematocrit are used to calculate the average amount of hemoglobin within each red blood cell, the mean corpuscular hemoglobin (mch); and its concentration, the mean corpuscular hemoglobin concentration (mchc). another calculation, the red blood cell distribution width (rdw), is derived from the standard deviation of the mean cell volume and reflects variation in cellular size. example of a white blood cell differential scattergram: differently coloured clusters indicate different cell populations after being treated with reagents, white blood cells form three distinct peaks when their volumes are plotted on a histogram. these peaks correspond roughly to populations of granulocytes, lymphocytes, and other mononuclear cells, allowing a three-part differential to be performed based on cell volume alone. more advanced analyzers use additional techniques to provide a five- to seven-part differential, such as light scattering or radiofrequency analysis, or using dyes to stain specific chemicals inside cells—for example, nucleic acids, which are found in higher concentrations in immature cells or myeloperoxidase, an enzyme found in cells of the myeloid lineage. basophils may be counted in a separate channel where a reagent destroys other white cells and leaves basophils intact. the data collected from these measurements is analyzed and plotted on a scattergram, where it forms clusters that correlate with each white blood cell type. another approach to automating the differential count is the use of digital microscopy software, which uses artificial intelligence to classify white blood cells from photomicrographs of the blood smear. the cell images are displayed to a human operator, who can manually re-classify the cells if necessary. most analyzers take less than a minute to run all the tests in the complete blood count. because analyzers sample and count many individual cells, the results are very precise. however, some abnormal cells may not be identified correctly, requiring manual review of the instrument's results and identification by other means of abnormal cells the instrument could not categorize. point-of-care testing point-of-care testing refers to tests conducted outside of the laboratory setting, such as at a person's bedside or in a clinic. this method of testing is faster and uses less blood than conventional methods, and does not require specially trained personnel, so it is useful in emergency situations and in areas with limited access to resources. commonly used devices for point-of-care hematology testing include the hemocue, a portable analyzer that uses spectrophotometry to measure the hemoglobin concentration of the sample, and the i-stat, which derives a hemoglobin reading by estimating the concentration of red blood cells from the conductivity of the blood. hemoglobin and hematocrit can be measured on point-of-care devices designed for blood gas testing, but these measurements sometimes correlate poorly with those obtained through standard methods. there are simplified versions of hematology analyzers designed for use in clinics that can provide a complete blood count and differential. manual manual determination of hematocrit. the blood has been centrifuged, separating it into red blood cells and plasma. the tests can be performed manually when automated equipment is not available or when the analyzer results indicate that further investigation is needed. automated results are flagged for manual blood smear review in 10–25% of cases, which may be due to abnormal cell populations that the analyzer cannot properly count, internal flags generated by the analyzer that suggest the results could be inaccurate, or numerical results that fall outside set thresholds. to investigate these issues, blood is spread on a microscope slide, stained with a romanowsky stain, and examined under a microscope. the appearance of the red and white blood cells and platelets is assessed, and qualitative abnormalities are reported if present. changes in the appearance of red blood cells can have considerable diagnostic significance—for example, the presence of sickle cells is indicative of sickle cell disease, and a high number of fragmented red blood cells (schistocytes) requires urgent investigation as it can suggest a microangiopathic hemolytic anemia. in some inflammatory conditions and in paraprotein disorders like multiple myeloma, high levels of protein in the blood may cause red blood cells to appear stacked together on the smear, which is termed rouleaux. some parasitic diseases, such as malaria and babesiosis, can be detected by finding the causative organisms on the blood smear, and the platelet count can be estimated from the blood smear, which is useful if the automated platelet count is inaccurate. to perform a manual white blood cell differential, the microscopist counts 100 cells on the blood smear and classifies them based on their appearance; sometimes 200 cells are counted. this gives the percentage of each type of white blood cell, and by multiplying these percentages by the total number of white blood cells, the absolute number of each type of white cell can be obtained. manual counting is subject to sampling error because so few cells are counted compared with automated analysis, but it can identify abnormal cells that analyzers cannot, such as the blast cells seen in acute leukemia. clinically significant features like toxic granulation and vacuolation can also be ascertained from microscopic examination of white blood cells. the hematocrit can performed manually by filling a capillary tube with blood, centrifuging it, and measuring the percentage of the blood that consists of red blood cells. this is useful in some conditions that can cause automated hematocrit results to be incorrect, such as polycythemia (a highly elevated red blood cell count) or severe leukocytosis (a highly elevated white blood cell count, which interferes with red blood cell measurements by causing white blood cells to be counted as red cells). left: a modified fuchs-rosenthal hemocytometer. right: view through the microscope of the hemocytometer. the built-in grid helps to keep track of which cells have been counted. red and white blood cells and platelets can be counted using a hemocytometer, a microscope slide containing a chamber that holds a specified volume of diluted blood. the hemocytometer's chamber is etched with a calibrated grid to aid in cell counting. the cells seen in the grid are counted and divided by the volume of blood examined, which is determined from the number of squares counted on the grid, to obtain the concentration of cells in the sample. manual cell counts are labour-intensive and inaccurate compared to automated methods, so they are rarely used except in laboratories that do not have access to automated analyzers. to count white blood cells, the sample is diluted using a fluid containing a compound that lyses red blood cells, such as ammonium oxalate, acetic acid, or hydrochloric acid. sometimes a stain is added to the diluent that highlights the nuclei of white blood cells, making them easier to identify. manual platelet counts are performed in a similar manner, although some methods leave the red blood cells intact. using a phase-contrast microscope, rather than a light microscope, can make platelets easier to identify. the manual red blood cell count is rarely performed, as it is inaccurate and other methods such as hemoglobinometry and the manual hematocrit are available for assessing red blood cells; but if it is necessary to do so, red blood cells can be counted in blood that has been diluted with saline. hemoglobin can be measured manually using a spectrophotometer or colorimeter. to measure hemoglobin manually, the sample is diluted using reagents that destroy red blood cells to release the hemoglobin. other chemicals are used to convert different types of hemoglobin to one form, allowing it to be easily measured. the solution is then placed in a measuring cuvette and the absorbance is measured at a specific wavelength, which depends on the type of reagent used. a reference standard containing a known amount of hemoglobin is used to determine the relationship between the absorbance and the hemoglobin concentration, allowing the hemoglobin level of the sample to be measured. in rural and economically disadvantaged areas, available testing is limited by access to equipment and personnel. at primary care facilities in these regions, testing may be limited to examination of red cell morphology and manual measurement of hemoglobin, while more complex techniques like manual cell counts and differentials, and sometimes automated cell counts, are performed at district laboratories. regional and provincial hospitals and academic centres typically have access to automated analyzers. where laboratory facilities are not available, an estimate of hemoglobin concentration can be obtained by placing a drop of blood on a standardized type of absorbent paper and comparing it to a colour scale. quality control automated analyzers have to be regularly calibrated. most manufacturers provide preserved blood with defined parameters and the analyzers are adjusted if the results are outside defined thresholds. to ensure that results continue to be accurate, quality control samples, which are typically provided by the instrument manufacturer, are tested at least once per day. the samples are formulated to provide specific results, and laboratories compare their results against the known values to ensure the instrument is functioning properly. for laboratories without access to commercial quality control material, an indian regulatory organization recommends running patient samples in duplicate and comparing the results. a moving average measurement, in which the average results for patient samples are measured at set intervals, can be used as an additional quality control technique. assuming that the characteristics of the patient population remain roughly the same over time, the average should remain constant; large shifts in the average value can indicate instrument problems. the mchc values are particularly useful in this regard. in addition to analyzing internal quality control samples with known results, laboratories may receive external quality assessment samples from regulatory organizations. while the purpose of internal quality control is to ensure that analyzer results are reproducible within a given laboratory, external quality assessment verifies that results from different laboratories are consistent with each other and with the target values. the expected results for external quality assessment samples are not disclosed to the laboratory. external quality assessment programs have been widely adopted in north america and western europe, and laboratories are often required to participate in these programs to maintain accreditation. logistical issues may make it difficult for laboratories in under-resourced areas to implement external quality assessment schemes. included tests the cbc measures the amounts of platelets and red and white blood cells, along with the hemoglobin and hematocrit values. red blood cell indices—mcv, mch and mchc—which describe the size of red blood cells and their hemoglobin content, are reported along with the red blood cell distribution width (rdw), which measures the amount of variation in the sizes of red blood cells. a white blood cell differential, which enumerates the different types of white blood cells, may be performed, and a count of immature red blood cells (reticulocytes) is sometimes included. red blood cells, hemoglobin, and hematocrit main articles: red blood cell, hemoglobin, hematocrit, and red blood cell indices sample cbc in microcytic anemia analyte result normal range red cell count 5.5 x 1012/l4.5–5.7 white cell count 9.8 x 109/l4.0–10.0 hemoglobin 123 g/l133–167 hematocrit 0.420.35–0.53 mcv 76 fl77–98 mch 22.4 pg26–33 mchc 293 g/l330–370 rdw 14.5%10.3–15.3 an example of cbc results showing a low hemoglobin, mcv, mch and mchc. the person was anemic. the cause could be iron deficiency or a hemoglobinopathy. red blood cells deliver oxygen from the lungs to the tissues and on their return carry carbon dioxide back to the lungs where it is exhaled. these functions are mediated by the cells' hemoglobin. the analyzer counts red blood cells, reporting the result in units of 106 cells per microlitre of blood (× 106/μl) or 1012 cells per litre (× 1012/l), and measures their average size, which is called the mean cell volume and expressed in femtolitres or cubic micrometres. by multiplying the mean cell volume by the red blood cell count, the hematocrit (hct) or packed cell volume (pcv), a measurement of the percentage of blood that is made up of red blood cells, can be derived; and when the hematocrit is performed directly, the mean cell volume may be calculated from the hematocrit and red blood