The packed cell volume (PCV) and total protein (TP) is a simple yet informative laboratory test, but one that is often misinterpreted or under-utilised.
It is important to remember that all test results need to be interpreted in the light of the patient’s history, presenting clinical signs and general physical examination findings.
Below is a table of the various changes that can be found on a PCV/TP and the possible causes of these changes. Many of the differentials can be include or excluded based on the history, clinical signs and examination findings.
I would like to highlight some common misconceptions that I find with PCV/TP interpretation:
1) A normal PCV/TP means the patient cannot be dehydrated.
The concept that all dehydrated patients will have an elevated PCV/TP is inaccurate. Patients will have to be severely dehydrated to see an elevation in both PCV/TP. Dehydration should be based primarily on physical examination findings, not based only on PCV/TP results.
2) A patient with a normal PCV could not have lost blood as the PCV should be low.
Patients can have acute whole blood loss which is not reflected in the PCV at presentation. This could have been caused by a number of reasons: the extravascular fluid has not yet shifted down the hydrostatic pressure gradients; the patient has not ingested water since the time of blood loss; or that IV fluid has not been given to correct the hypovolaemia. Once the fluid shifts, the patient drinks water or IV fluids are administered, there is be a drop in the PCV/TP due to haemodilution.
3) An elevated PCV means that patient is dehydrated.
This is probably the most common change I see in my patients and it is not because they are all dehydrated. The most common cause of this change is stress-induced splenic contraction. The spleen stores red blood cells. Under the influence of adrenalin, the smooth muscle in the spleen contracts and the stored red blood cells are pushed into circulation. The next most common cause would be haemorrhagic gastroenteritis where there has been a fluid shift into the gastrointestinal tract.
Also, assessment of the serum colour can provide valuable information. White or lipaemic serum can cause artifactually high TP. Haemolysed serum with a low PCV can indicate a haemolytic anaemia.
Next time you perform a PCV/TP have look at this table and consider these other differentials for your results.