The aspirate collected in EDTA should be sent to the laboratory for processing. After proper mixing (inverting the tube gently several times), the aspirate sample should be pipetted into a 1 mL Wintrobe tube, assuring that there are no air bubbles. Place the Wintrobe tube in a centrifuge and spin at 2800-3000 rpm for 8-10 minutes. After centrifugation, the sample is separated into four distinct layers. From the bottom to the top of the aspirate in the Wintrobe tube (shown in the image on the right) are erythrocytes (E)/red blood cells (RBCs), myeloid-erythroid (M-E) cells (buffy coat), plasma (P), and the fat and perivascular (F-PV) cells.
The bottom layer (RBCs) and clear layer (plasma) will vary, depending on the individual's blood dilution. The F-PV layer is rich in histiocytes and is useful for demonstrating marrow iron stores. The volume of each layer is measured using the scale on the Wintrobe tube and then the percentage of each layer is calculated. Record the percentages (1 mL equals 100%) for the pathologist, as shown in the image below. This is sometimes referred to as the bone marrow "crit." Occasionally, the percentages are difficult to discern. Re-centrifugation might help.
