Feline bloodwork is a common diagnostic tool used in the feline patient. It is often ordered as a geriatric screening for older cats that appear healthy, a pre-anesthesia screening, or as part of a set of diagnostics in systemically ill patients. As a geriatric screening, systemic imbalances such as hyperthyroidism or early liver or kidney disease can be caught before symptoms are present. Pre-anesthetic screenings are critical in the geriatric patient because most anesthetics are cleared by the liver and/or kidney, and can be life threatening in compromised patients. In a systemically ill patient, bloodwork can be essential in the process of ruling in or ruling out disease and assisting in the narrowing of the differential diagnoses list. It is unusual for bloodwork alone to provide a definitive diagnosis, but combined with the total clinical picture it can be extremely informative.
Furthermore, even when bloodwork does not provide a primary diagnosis, it can often provide information about secondary imbalances that can be considered treatable symptoms (such as electrolyte imbalances). For a basic example, a vomiting cat may have low potassium secondary to vomiting and anorexia (not eating). The low potassium may lead to a set of tertiary symptoms, including muscle weakness, tachycardia, or arrhythmia, all of which can be immediately life threatening. In addition, the secondary and tertiary symptoms can mask the underlying cause of vomiting, and make it difficult to tell what the primary problem is. Running labs, in this scenario, could give the veterinarian enough information to treat the hypokalemia and potentially eliminate life threatening arrhythmias even before the primary diagnosis is made. Equally importantly, the complicating symptoms may be removed from the problem list to help paint the primary clinical picture and clarify the possible primary cause.
The purpose of this paper is to give a general overview of the two most commonly ordered blood tests, the Chemistry Profile and the Complete Blood Count (CBC). Simplified tables are provided with the most common values included on these tests, what normal values generally look like, and some notes about what body systems are generally responsible for changes in values. In many cases, of course, such as electrolytes (sodium, potassium, chloride), virtually any system imbalance can lead to secondary or tertiary abnormalities. In order to determine the primary cause of an abnormal value, the total clinical picture MUST be considered all together. In addition, multiple blood values must be considered together, because one often influences another, and because many diseases tend to display a pattern. Although it is a complex topic, there are some generalizations that can be made to help understand what some of the values are telling us.
The most common values included in a Profile are shown in Table 1. The first two values, BUN and creatinine, are key values to indicate kidney function and glomerular filtration rate (GFR). BUN and creatinine are cleared by the kidney. If they are high, the patient is azotemic. Azotemia means BUN and creatinine are high, as well as phosphorous in some cases. Azotemia does not necessarily mean primary renal failure, however. Azotemic patients are not clearing BUN and creatinine for one of three reasons: pre-renal, renal, or post-renal. Pre-renal azotemia means that the patient is dehydrated. Dehydration leads to decreased blood volume, which results in decreased blood flow to the kidney, and therefore decreased GFR. Pre-renal azotemia is corrected by giving the patient fluids. In post-renal azotemia, the problem is between the kidney and the final excretion of urine, such as a ruptured bladder or ureters. In post-renal azotemia, potassium is also usually elevated because it is trapped in the body, even though the kidneys are functioning normally. In order to diagnose primary renal azotemia (kidney failure), pre and post renal azotemia must be ruled out.
Phosphorous and calcium are influenced by a complex set of variables. Both are influenced by diet and bone growth, and both are influenced by each other by a complex feedback system. One of the main considerations for these values is the parathyroid gland and its role in calcium control. Parathyroid hormone (PTH) influences the amount of calcium that is taken from bone and also the amount that is absorbed from the diet. In addition, vitamin D production by the kidney is influenced by PTH, and is necessary in the absorption of calcium from the diet. To further complicate matters, a large portion of calcium exists in the blood bound to albumin. If a patient has a protein losing nephropathy (kidney disease), a protein losing enteropathy (GI disease), or a hemorrhage, then albumin is being lost and calcium goes with it.
The electrolytes (sodium, potassium, chloride) can be affected by any body system. However, it is useful to know that the kidney plays a central role in the balance of electrolytes, especially sodium and potassium. For example, a severely dehydrated patient will have very concentrated urine if the kidney is functioning as it should. Aside from the kidney, however, the feline patient has a tendency to lose potassium faster than most species in the face of vomiting or anorexia. It is helpful to realize that the only source of potassium is through the diet. If a cat is not eating it and is also vomiting it up, the body can be depleted quickly. Felines also lose chloride while vomiting, because the chloride present in the stomach (HCl) is being expelled, and this may result in acid base imbalance (revealed in bicarbonate and anion gap values). Electrolytes are critical to proper nerve function and myocyte function in the body.
AST, ALT, ALP, CK, and cholesterol are a group of tests that help evaluate liver function. AST, ALT, and ALP are hepatocellular enzymes that are released into the blood when there is direct cellular damage to liver cells. AST and ALP are fairly nonspecific, because they can also be released from muscle or other tissue damage. ALT is fairly liver specific. CK helps differentiate, because it is only released from muscle damage (interestingly, CK can be high in human patients after a heart attack). If the only enzymes in this group that are elevated are AST and CK, then it is generally concluded that the liver is probably okay and the cause is muscle damage. However, if the pattern of elevated values includes AST, ALT, and ALP, but CK is normal, it would suggest that liver damage is more likely. If the only value that is elevated is ALP, that is considered very non-specific and not very informative. Finally, if cholesterol is abnormal, it may be due to liver dysfunction.
Bilirubin can also indicate hepatic dysfunction, but not always. Bilirubin is one of the products of normal red blood cell destruction in the body. As RBC's age, they eventually need to be cleared and the iron recycled to make new RBC's. When bilirubin is produced, it goes to the liver to go through a conjugation process, which prepares it to be able to be cleared from the body. If bilirubin is not being cleared, that usually indicates one of two things. One option is that the liver is unable to conjugate the bilirubin because the liver is not functioning properly. Another option is that there is an overwhelming amount of bilirubin that needs to be conjugated and a normal liver cannot keep up. This might be due to excessive RBC destruction, such as hemolytic anemia, which is an autoimmune problem where the body attacks its own RBC's. There are also oxidative damage problems, such as a toxic reaction to onions, that can lead to hemolytic anemia and an increase in bilirubin. In a feline patient with high bilirubin, a urine test would also be indicated. Bilirubin present in canine urine can be normal, but it is never normal in the feline patient.
Total protein is a very important value, but on its own is not very informative. Protein in the blood has multiple functions. The most important function of protein in the blood stream is to maintain an osmotic gradient to keep the blood hydrated. Without protein in the blood, the water content would be lost from the vessels. Patients would develop edema and the total blood volume would decrease, which would lead to kidney failure. Diagnostically, there are three basic causes for a decrease in total protein: 1) lose it from the gut (enteropathy), 2) lose it from the kidney (nephropathy), 3) not producing it (liver disease).
Glucose is another value that is not all that helpful on its own, but can be extremely useful when combined with a urinalysis. If glucose is extremely high on the profile and is also present in the urine, that is strongly suggestive of diabetes.
CBC values are listed in Table 2. Total protein on a CBC is slightly different from that of the profile because it is run on plasma, while the protein measurement on a profile is run on serum, but are otherwise similar. Packed cell volume (PCV or hematocrit) refers to the percentage of red blood cells present in a volume of whole blood. A low PCV usually indicates anemia, either from loss of red blood cells or lack of production of red blood cells. Severe hemorrhage from major trauma such as a hit by car can cause loss of both RBC's and proteins, making both values low. However, if total protein and PCV are both high, this usually indicates dehydration. Loss of water will simply increase the ratio of protein and RBC's per volume of whole blood, since the cells and proteins exist in a river of water. If the water is drained, then the density increases due to a higher proportion of cellular material. If the PCV is very high, but the protein is normal, the patient may be polycythemic. Polycythemia is a condition where the body produces excess amounts of red blood cells, usually secondary to hypoxia. The body may respond to a need for oxygen by producing more blood cells to carry it. High altitude can be a factor. If a patient is Polycythemic, the PCV, HGB, and RBC counts should all be very high.
HGB (hemoglobin), RBC, MCV, and MCHC provide information regarding the size and content of RBC's, which can tell something about the production process of RBC's from the bone marrow and the process of loss in the case of anemia. Young RBC's are larger than old RBC's. Lack of hemoglobin content could indicate a lack of iron because iron is needed to build the hemoglobin complex in the cell. High MCHC (mean cellular hemoglobin content) usually means there is excessive free HGB in the blood vessels, due to an increase in RBC destruction which liberates the HGB. If that is the case, it stands to reason that the bilirubin value on the profile would also be increased, and the patient may be icteric. Recall that bilirubin is a product of RBC destruction. It also has a yellow pigment which can be seen in the plasma and also can turn the patient's membranes yellow.
Platelets are necessary for proper blood clotting function, along with several other necessary factors. It is important to realize that clotting is happening all the time inside the body. Without clotting function, a patient may bleed out of small nicks and damage to endothelial cells inside blood vessels. Without platelets, a patient would be at high bleeding risk even without major trauma causing visible bleeding. This value is very frequently considered before subjecting a patient to any surgery. For example, a geriatric patient with liver disease may need to have a liver biopsy taken. But if the patient has very low platelets or any other clotting disorder, the process of taking tissue from the highly vascular organ could cause the patient to bleed to death. Increased platelets (thrombocytosis) is usually due to inflammation or iron deficiency.
White blood cells generally indicate inflammation or infection somewhere in the body when they are elevated. Different types of WBC's are produced in the body to handle different types of infections, although there is quite a bit of crossover. Low WBC counts may indicate that the body is unable to produce enough cells to keep up with an infection. Band neutrophils are the youngest form of the neutrophil, and indicate that the bone marrow is trying to produce and release cells as quickly as possible. A few bands can be present in a normal patient, but an overload of bands always means that there is an active infection. If a patient has a significant quantity of eosinophils, a parasitic infection may be suspected. High lymphocyte counts may be present in viral infections or lymphoma.
Subjective evaluation of blood smears under the microscope can also be revealing. Toxic changes to white cells sometimes means the cells are old and not being cleared by the body as quickly as they should. Changes to red cells sometimes reveals oxidative damage or suggests extra production of cells from the bone marrow. Abnormal blood smears are often evaluated by a board certified pathologist because the vast spectrum of processes that can cause abnormalities including certain cancers, autoimmune diseases, parasites, poisons, viruses, fungal infections, toxins, and more.
Feline patients present many challenges. Since they are easily stressed, there are a few diagnostics that are especially difficult to evaluate in the cat. For example, it was once believed that feline blood pressure averaged higher than most other species in a healthy patient. Eventually, someone found a way to measure blood pressure remotely as part of an experiment to determine what normal feline blood pressure is when the cat is completely relaxed at home. As it turns out, feline blood pressure is similar to most other species, but the moment they are stressed, such as being handled by a stranger, it goes up. This has become known as the white coat artifact.
Glucose also has a tendency to show dramatic increases in a healthy cat under acute stress. Normal blood glucose should be well below 150 mg/dl. In most species, a glucose measurement of over 200 would strongly suggest diabetes. In the feline, however, stress alone can cause glucose to spike as high as 250 mg/dl. A value over 300 is unlikely to be due to stress alone. One way to differentiate between a glucose spike due to acute stress and high blood glucose due to diabetes is through urinalysis. If there is no glucose in the urine in the face of high blood glucose, it may be due to the fact that the blood glucose has not been high long enough to overflow into the urine, suggesting an acute spike. This is not definitive, however. Presence of glucose in the urine is, however, highly suggestive of diabetes.
Another stress response that is often seen in cats is referred to as a stress leukogram. In a stress leukogram, the differential white blood cell count becomes skewed. It is usually possible to recognize this by the pattern of abnormalities, but can sometimes mask underlying abnormalities and make it difficult to tell what the real values should be. Two stress leukogram patterns are typical. A sick cat under chronic stress has increased glucocorticoid release, which leads to a decrease in lymphocytes and an increase in neutrophils and monocytes. This type of leukogram can also be present in a patient being medicated with glucocorticoids. An acutely stressed cat, such as a fractious cat afraid at the vet clinic, releases epinephrine. This is the type of stress that increases glucose. It also increases lymphocytes and neutrophils. Both of these patterns can be difficult to interpret. One value in the leukogram that cannot be mistaken, however, is the band neutrophil count. A high band count always indicates active infection. Bands are young neutrophils, and are released from the bone marrow at a high rate in response to an active infection. The only exception to this rule is a rare genetic anomaly known as Pelger-Huet, which causes all neutrophils in the blood to appear immature. Pelger-Huet is recognized because of the complete absence of mature neutrophils.
Previously in this paper, polycythemia was discussed. One of the stress responses common in both cats and horses is an epinephrine driven splenic contraction. The spleen serves as a storage place for an immediate, emergency supply of RBC's. Epinephrine causes the spleen to contract and release RBC's, which can increase the PCV without an increase in total protein, and can look suspiciously like polycythemia. It is unusual for this response to be dramatic enough to fool the veterinarian, but should be considered in an exceptionally fearful cat, such as a feral cat being restrained for the first time.