February 2025

In this VETgirl online veterinary continuing education blog, Amanda Shelby, RVT, VTS (Anesthesia & Analgesia) reviews the wealth of information that can be gained from the simple and routinely performed packed cell volume (PCV)/total protein (TP) diagnostic test, and how this information can impact your patient care plan.

Performing a PCV/TP is a foundation skill for veterinary technicians. Along with a patient’s physical examination and history, a veterinarian often requests PCV/TP to be performed alongside other diagnostic blood work. In context, PCV/TP can provide the veterinary team important information about the patient within just a few minutes. But before we detail the specifics of what a PCV/TP may tell us about a patient, let’s quickly review what the PCV/TP is and what supplies are needed to perform this valuable diagnostic test (see Table 1, Figure 1 and 2). For more information on how to perform a PCV/TP, check out this VETgirl veterinary continuing education video, or you can refer to Table 2 for step-by-step instructions.

PCV reflects the portion of red blood cells within a blood sample and is measured as a percentage of whole blood. You may have seen ‘PCV’ used interchangeably with ‘HCT’ which stands for hematocrit. While both represent the percentage of red blood cells in a blood sample, PCVs are performed manually, whereas HCTs are performed by laboratory analyzers. Theoretically, a blood sample would have the same PCV as HCT; however, small difference may be seen. Different testing variables can influence either method: PCV may be slightly elevated compared to HCT due to a small fraction of plasma trapped around the red blood cells after centrifugation; lab analyzers can be affected by variables in the blood sample such as lipemia and hyperbilirubinemia. Generally, any difference between the two is typically small.

Proteins reside in the plasma portion of blood and are measured as TP with a refractometer using a drop of plasma from a centrifuged blood sample. TP (total proteins) and TS (total solids) are often thought of as being the same thing…but they’re not exactly. (If you want to learn more about the difference between TS and TP, check out this VETgirl continuing education blog).

Table 1. Supplies for Performing a PCV/TP

  • Syringe/needle
  • Microhematocrit tube (heparinized [red stripe] – used for fresh whole blood samples, non-heparinized [blue stripe] – used for anticoagulated blood samples)
  • Centrifuge
  • PCV chart or metric ruler (with millimeter graduations)
  • Clay
  • Refractometer

 

Figure 1. Microhematocrit centrifuge. Photo courtesy of Amanda M. Shelby

 

 

Figure 2. PCV/TP supplies. Photo courtesy of VETgirl

 

Table 2. Performing a PCV/TP

*VETgirl Protip: Don’t have a chart? check out this VETgirl veterinary continuing education video on How to Calculate a PCV without a Microhematocrit Capillary Tube Reader.

**Many refractometers have TP scales on the left and specific gravity scales on the right, but this can vary between manufacturers so always look for the correct units. Most commonly, TP is measured and recorded in g/dL in America, but may be recorded as g/L in other countries.

Normal Ranges

There are a handful of variables we need to consider when determining what should be ‘normal’ PCV/TS values for our patient. There are species variations in normal references ranges for PCV/TP.  Normal reference ranges vary with age; puppies and kittens have lower PCV/TP than healthy adults. Pregnant patients have lower PCVs than non-pregnant, comparably healthy, patients due to an increase in the fluid component (plasma) of blood during pregnancy causing hemodilution. Patients living at higher altitudes typically have a higher PCV as compared to those living at sea level. Even the normal color of plasma can be different among species; yellow plasma is normal in horses, whereas in cats and dogs this would be considered an abnormal finding- what we call “icterus.” Table 3 provides normal PCV/TS ranges for some common species.

So, what information about our patient can PCV/TP provide?

Hydration Status: An elevated PCV/TP could provide an indication that our patient is dehydrated. In this condition, both PCV and TP should increase in a parallel manner (e.g., hemoconcentration). As the total body fluid decreases in due to dehydration, our water moves out of the vascular space (out of the plasma portion of the blood) and into the extracellular spaces and cells. This causes a decrease in plasma volume, shifting the ratio of red blood cells:plasma – thus the PCV increases in dehydration. Similarly, the movement of water out of the blood and into our dehydrated tissues effectively concentrates the remaining plasma proteins – thus the TP increases as well in dehydration. Despite this being a fairly reliable trend, exceptions always exist, and so a diagnosis of ‘dehydration’ requires evaluation of the whole patient by the veterinarian and should not be made off of PCV/TP alone.

Presence of Anemia/Hemorrhage: A low PCV with or without a low TP can be one indicator of anemia. Likewise, a normal to only mildly low PCV (e.g., 30-35%) with a low TP can be an indicator of acute anemia from hemorrhage (e.g., hemoabdomen). Detection of a sudden decrease in TP could indicate ongoing or massive hemorrhage— this can be a helpful diagnostic tool during or immediately following a complicated surgery.1 A full patient assessment is still required before a veterinarian can determine a diagnosis. An anemic, dehydrated patient could have a ‘normal’ PCV value due to hemoconcentration; once the patient is rehydrated, the PCV will decrease with hemodilution from the fluid therapy, unmasking the underlying anemia. In addition to a typically lower PCV, anemic patients may also have pale mucous membranes on physical examination, possibly tachycardia, and potentially other blood work abnormalities on a complete blood count (CBC).

Presence of Inflammatory State: A normal PCV with an elevated TP can be seen with “inflammatory” diseases such as Feline Infectious Peritonitis (FIP), chronic inflammation (e.g., severe gingival disease or skin disease), or diseases resulting in abnormally high protein production (e.g., multiple myeloma).

Diagnosis of Hemoabdomen/Hemothorax: When fluid is obtained from a body cavity (e.g., thorax, pericardial sac, abdomen), a PCV should be performed on the fluid; this fluid should NEVER clot unless a vital organ was punctured (e.g., heart, spleen). The PCV of the effusion can yield helpful information regarding ongoing bleeding (e.g., low PCV more consistent with chronic bleeding, “normal” PCV more consistent with acute bleeding). This often dictates the degree of urgency to resolve the source of the hemorrhage (e.g., hemoabdomen secondary to splenic hemangiosarcoma resulting in acute ongoing hemorrhage).

Estimation of Blood Loss: When a patient has lost >20% of their total blood volume, or has clinical signs of hemorrhagic shock that are non-responsive to fluid therapy (e.g., crystalloids/colloids), a blood transfusion is recommended. The equations for estimating normal total blood volume in cats, dogs, and horses can be found in Figure 3. When excessive hemorrhage occurs during surgery and is collected into a suction jar, the equation in Table 4 can be used to estimate the patient’s blood loss from the diluted blood in the suction jar. The anesthetist should also observe the surgical area for blood-soaked sponges, blood splatters on the floor, and blood leaking underneath the surgical drapes. Guidance for assigning estimated blood volumes to these difficult-to-quantify sources of hemorrhaged blood can be found in Table 4.

Abnormal color of plasma): In addition to the PCV, assessment of the plasma portion of the microhematocrit tube should be noted. As mentioned above, there are some species differences in normal plasma color. Horses’ plasma is normally light to straw yellow to even light amber. If the plasma is white, milky or creamy, it indicates your patient is lipemic (see Figure 4). Lipemia can be seen in patients with hypercholesterolemia, hypertriglyceridemia (e.g., miniature schnauzer), in patients with pancreatitis, or following administration of lipids (e.g., intravenous lipid emulsion, partial or total parenteral nutrition). Patients with lipemia will have a falsely elevated TP. Severe cases can make it challenging to obtain a TP with a refractometer.

 

Figure 4. Lipemic microhematocrit tube sample. Photo courtesy of Erin Hunt, RVT

If the plasma is pink to red, this could indicate hemolysis (from iatrogenic sample handling or blood draw technique) or red blood cell destruction (from life-threatening diseases such as immune-mediated hemolytic anemia or even zinc toxicity) (see Figure 5). If the plasma is icteric, this indicates hyperbilirubinemia. The patient may have an underlying hepatopathy, immune-mediated hemolytic anemia, or a cause for hyperbilirubinemia (e.g., pancreatitis resulting in gallbladder or bile duct obstruction, gallbladder mucocele, gallbladder stones, neoplasia).

 

Figure 5. Example of hemolysis in microhematocrit tubes. Photo courtesy of Amanda M. Shelby

 

Figure 6. Icteric microhematocrit tube sample. Photo courtesy of Erin Hunt

Low protein: Conversely, a low TP (i.e., hypoproteinemia) could indicate a patient has a protein synthesis disruption (e.g., hepatic failure), protein-losing disease (e.g., enteropathy or nephropathy), or hemorrhage (see above). Low TP could lead to low oncotic pressures and increased permeability of cellular membranes leading to edema or even effusion.

As you have likely noticed, it is difficult to make a definitive diagnosis exclusively from a PCV/TP but when coupled with a complete physical examination and patient history (alongside diagnostic tests), it makes for a more comprehensive picture of patient condition. Having this simple, quick, and affordable diagnostic test available can provide the veterinary care team valuable information when providing individualized, tailored patient care plans.

References:

  1. Getzen, L. C. (1977). Serum protein concentration during hemorrhagic shock. Journal of Trauma and Acute Care Surgery, 17(3), 257.
  2. Shelby, AM, McKune, CM. Small Animal Anesthesia Techniques. New York:John Wiley & Sons; 2014 p. 204.

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