Red blood cell analysis

Complete blood count

One of the most common hematology tests ordered is the CBC. The CBC is often used as a screening test to determine a patient’s health status. It is used to screen for a variety of diseases and conditions, such as anemia, infection, and bleeding disorders. The CBC can be used to monitor the status of a condition. If a patient is undergoing chemotherapy, a CBC can also provide information on the impact of the treatment on the blood cells.

An automated hematology analyzer has been approved for CLIA-waived testing. This allowed CBCs to be done in the POL. If the results are abnormal or if the POL does not have the analyzer, the CBC is done in the reference (referral) laboratory. With abnormal results, a blood smear is examined using a microscope to identify additional information.

Collecting a specimen for a complete blood count

For a CBC, venous blood is collected in a lavender-topped tube containing ethylenediaminetetraacetic acid (EDTA), an anticoagulant that prevents whole blood from clotting. EDTA is the anticoagulant of choice for hematology testing because it also acts as a preservative for the blood cells. It is very important to prevent blood from being hemolyzed during collection for hematology testing.

Components of a complete blood count

The CBC includes cell counts and provides information on the physical features of some of the cells. A standard CBC includes RBC, WBC, and platelet tests. The following sections discuss the components of the CBC.

CLIA-waived complete blood count

CLIA approved the Sysmex XW-100 for CLIA-waived CBC tests. This analyzer performs a three-part differential and includes the following:

• Total number of RBCs, WBCs, and platelets
• Hemoglobin and hematocrit
• Mean corpuscular volume (MCV)
• Total number and percentage of neutrophils, lymphocytes, and other WBCs

Red blood cell tests

Part of the CBC incorporates RBC tests, including an RBC count, hematocrit, hemoglobin, and RBC indices.

Red blood cell count

The RBC count approximates the number of circulating RBCs in a person’s blood. The function of an RBC is to transport oxygen to tissues and help carry some carbon dioxide away from the cells. RBCs have a life span of about 120 days.

The RBC count often is decreased in patients with anemia. Increased RBC counts are found in people with dehydration, polycythemia, severe burns, and those living at high altitudes. The normal RBC values range from approximately 4.4 million to 5.7 million/mm3 in males and 3.9 million to 5.1 million/mm3 in females.

Reticulocyte count

Immature RBCs, also called reticulocytes, are RBCs that are still developing. They are made in the bone marrow and then sent to the bloodstream. Two days after forming, they develop into mature RBCs. A reticulocyte count (retic count) is done at a reference laboratory and can be part of the CBC. The reticulocyte count measures the number of reticulocytes in the blood. The normal value range for a reticulocyte count is 0.5% to 2.5%. An abnormally low or high count can indicate disorders of the bone marrow, liver, and kidneys. The retic count is also used to diagnose specific types of anemia.

Hematocrit

The hematocrit (Hct) is a measurement of the percentage of packed RBCs in a volume of blood. A spun microhematocrit test is based on the principle of separating the cellular elements from plasma using a centrifuge.

Normal Hct values vary with sex and age. They range from a low of 36% in women to a high of 49% in men. Low microhematocrit values can indicate anemia or the presence of bleeding. High values may be caused by dehydration or polycythemia . Values can be influenced by physiologic, pathologic , and even geographic factors and by collection techniques. Normal Hct ranges are affected by a person’s geographic location. For example, people living at high altitudes have a higher percentage of RBCs to compensate for the lower oxygen levels in the atmosphere.

Drops of blood are collected from a capillary puncture into two capillary tubes that are placed in a specially designed microhematocrit centrifuge. Capillary tubes can also be filled with EDTA-anticoagulated blood from a lavender-topped vacuum tube. As required by the Occupational Safety and Health Administration (OSHA), capillary tubes must be safe. They are made of either plastic or plastic-coated glass to avoid sharps injuries. The most common type of microhematocrit tube is self-sealing at one end. A less used type is open ended on both ends. If the tube is self-sealing, it must be tilted upright, causing the blood sample to flow down the tube and come into contact with the seal, and then held in place for 15 to 30 seconds. The open-ended tubes must be sealed with special clay before centrifugation.

After centrifugation, the packed RBCs are at the bottom of the tube against the sealant, the WBCs and platelets are in the center buffy coat, and plasma is on top. The microhematocrit is determined by comparing the volume of RBCs to the total volume of the whole blood sample. The percentage is read by placing the tubes on a special microhematocrit reader. Some microhematocrit centrifuges have a built-in reading scale that reads the calibrated capillary tubes. Microhematocrit tests should be performed in duplicate and the average of the two results reported.

The microhematocrit is a commonly performed test requested by providers either separately or as part of the CBC. Because it is a simple procedure that requires only a small amount of blood, it is an ideal screening test and often is part of a routine physical examination. Quality assurance includes care and maintenance of the microhematocrit instrument.

Hemoglobin

Hemoglobin (Hgb) determination is another way to measure the oxygen-carrying capacity of blood. The hemoglobin concentration can be part of the CBC or an individual test.

Normal hemoglobin values vary throughout a lifetime. The normal values for an adult male are 13.2 to 16.6 g/dL, and an adult female’s range is 11.6 to 15 g/dL. Factors that affect the hemoglobin level include age, sex, diet, altitude, and existing disease states.

Hemoglobin and hematocrit tests are often performed together and are referred to as an H&H. A quick mental calculation should always be done before H&H results are reported: the hemoglobin value × 3 (± 3) should equal the hematocrit value. For example, if the hemoglobin is 15 g/dL, calculate the hematocrit as follows:

{`	15 x 3= 45 +3 – 42 –48	
`}

The hematocrit should be between 42% and 48%.

CLIA-waived methods include use of the STAT-Site M Hgb, HemoPointH2, and HemoCue, all portable, battery-operated hemoglobin analyzers that fit in the palm of the hand. The HemoCue uses plastic microcuvettes that contain reagents that lyse (break apart) the RBCs in the sample, releasing the hemoglobin. The hemoglobin reacts with the reagents and forms a color. The color is detected and measured in the instrument, producing a digital readout. Capillary, venous, or arterial blood can be used in the disposable microcuvette. The cuvettes have a long shelf life.

Red blood cell indices

A variety of calculations can be performed using the information obtained from the CBC. The red blood cell indices provide information about RBC disorders. The indices are used to classify anemias and to select additional tests that may help determine the cause of anemia. Red blood cell indices are also helpful to monitor the treatment of anemia. Because indices may change in response to treatment, they can be used as an indicator of how well the patient is responding to a treatment. The indices are mathematical calculations using the three red blood cell tests: Hct, Hgb, and the RBC count.

• Mean corpuscular volume (MCV): MCV = (Hct/RBC) × 10. The MCV is calculated to give the volume of the average RBC in a blood sample. The average size of the RBCs is the most important index for classifying anemias. Abnormal RBCs include these:
• Macrocytic: Abnormally large RBCs and have a higher-than-normal MCV.
• Microcytic: Small RBCs and have a lower-than-normal MCV. The normal reference range is 80 to 100 femtoliters (fL).
• Mean corpuscular hemoglobin (MCH): MCH = (Hgb/RBC) × 10. The MCH is calculated to give the average weight of hemoglobin in the RBC. The reference range is 26 to 34 picograms (pg).
• Mean corpuscular hemoglobin concentration (MCHC): MCHC = (Hgb × 100)/RBC. The MCHC indicates the average weight of hemoglobin compared with the cell size. A decreased MCHC shows hypochromic (pale, lacking color) RBCs in a stained blood smear. An increased MCHC is rare and should be flagged and brought to the attention of the provider.