Many people blow off simple quick assessment tests like the packed cell volume (PCV), total protein (TP), blood glucose (BG) and AZO (BUN). However, these $5 diagnostic tests can provide so much information, particularly in the veterinary emergency room or ICU!
[For more information on the difference between TP (total protein) and TS (total solids), check out our previous blog post
The PCV measures the percentage of red blood cells (RBC) to the total blood volume. This value is obtained when a blood sample is placed into a small hematocrit tube.
Following hematocrit tube filling, the tube is sealed with clay and placed in the centrifuge for approximately 3 minutes (for most standard 10,000-12,000 RPM centrifuges). Following centrifugation, the RBCs are separated from the plasma. Plasma is mostly comprised of water, proteins, glucose, clotting factors, and mineral ions.
When assessing the PCV and TP, it's important to evaluate these two parameters together.
When evaluating the PCV, the first step is to decide if the value is normal. While the results may vary slightly based on age and species, normal PCV is approximately 35% - 45%. While polycythemia (a disease state in which the proportion of blood volume that is occupied by red blood cells increases) is possible, most commonly an increase in PCV is seen with dehydration; this is commonly termed hemoconcentration. A more common clinical problem - at least in the veterinary ER or ICU - is the dilemma of determining why the patient is anemic (e.g., low PCV).
This is where evaluating the PCV in relation to the TP is handy!
The TP is measured by breaking the hematocrit tubes (once the PCV is determined) following centrifugation, and placing the plasma directly onto the refractometer. The refractometer is then assessed (pointing it into the direction of light) - check out the picture to see the reading. The TP is the number read directly from the visible scale (in this picture, a total protein of approximately 1.6).
Next, the color and clarify of the plasma should be assessed. If the plasma has a yellow-tinge, it typically represents an increase in bilirubin level. If the plasma has a red-wine color, the sample is typically hemolyzed (e.g., either due to traumatic venipuncture or direct hemolysis within the body, etc.). Lastly, the serum should be evaluated for the presence of hyperlipidemia (e.g., cloudy white plasma) or the presence of a large buffy coat (e.g., a thick, opaque white line found directly above the layer of RBC and plasma); this is typically due to presence of an elevated white blood cell count.
Similar to the PCV, the TP may vary slightly based on age and species, with normal values ranging from 6.5 - 8.0 gm/dl. Elevated TP is most commonly seen with dehydration, with less common causes being chronic inflammation, neoplasia, infectious disease (e.g., FIP), or multiple myeloma. A lower than normal protein level can be seen with hemorrhage, malabsorption, liver disease (e.g., lack of production of albumin), gastrointestinal disease (e.g., protein-losing enteropathy), or kidney disease (e.g., protein-losing nephropathy).
This chart is an example of how different combinations of PCV/TS can help narrow down disease quickly.
↓ PCV/ N TS
(i.e., 25%/70 g/L)
↑ PCV/N TS
(i.e., 65%/7 g/dL)
(i.e., 65%/70 g/L)
N PCV/↓ TS
(i.e., 40%/5 g/dL)
(i.e., 40%/50 g/L)
N PCV/↑ TS
(i.e., 40%/9 g/dL)
(i.e., 40%/90 g/L)
(i.e., 66%/8 g/dL)
(i.e., 66%/80 g/L)
↓ PCV/ ↓ TS
(i.e., 25%/5 g/dL)
||Protein-losing enteropathy (PLE)
||Chronic blood loss (Melena)
||Protein-losing nephropathy (PLN)
||Feline infectious peritonitis (FIP)
||Blood loss (subacute)
|Pure red blood cell aplasia
||Liver failure (lack of production of albumin)
||Chronic globulin stimulation (i.e., dental, skin disease)
|Anemia of chronic disease
||Hemorrhagic gastroenteritis (HGE)
||Acute blood loss (with splenic contraction)
||Severe dehydration + anemia (i.e., CRF)
|Is the sample hemolyzed? Icteric? èIMHA
||EPO-producing tumor (renal)