Last month we described how to interpret a CBC, which stands for complete blood count. It’s a blood test that looks at all your dog’s red and white blood cells to check for abnormalities that might indicate disease or explain a dog’s symptoms of illness. This month, we look at a second blood test that’s virtually as routine as the CBC. Called the chemistry profile, it analyzes the blood minus the red and white blood cells.
“We literally take out the cells and look at the stuff in what’s left,” say Your Dog editor-in-chief John Berg, DVM. “And what’s left is the serum, or fluid. It’s often collected in the same syringe as blood taken for the CBC,” he explains. “If you know in advance that you are running both tests,” he comments, “the sample for both comes from the same blood draw.” In fact, Dr. Berg notes, “when a vet says to a pet owner, ‘Let’s start by running some blood tests,’ he almost always means a CBC and a blood chemistry profile. A lot of things can be ruled out definitively just by those two tests.”
Adds Linda Ross, DVM, DACVIM, an internal medicine veterinarian at Tufts and a member of our editorial board, “going back to the dark ages when I graduated from veterinary school [the 1970s], we didn’t have these screenings available. If you wanted a test, you had to order a specific one. Today, we can get a whole panel, which allows us to provide much better care at hardly any more cost. We can detect problems we may not have even thought to consider.”
Here’s a rundown of the chemicals assessed in a chemistry profile and what they might signify.
Glucose: Most often, a high concentration of glucose, or blood sugar, means the dog has diabetes, although “sometimes glucose can be increased in dogs who are simply stressed,” says Dr. Berg. Glucose levels that turn out low can be the result of more possibilities. “Sometimes it’s just a laboratory issue,” Dr. Ross says. “If a blood sample sits too long, the level of glucose in it may fall. Red blood cells metabolize, or digest, the sugar.
“But there are diseases consistent with low concentrations of blood glucose as well, including insulinoma — the production of too much insulin because of, say, a pancreatic tumor. Low blood sugar can also be consistent with Addison’s disease and also with severe infections.
Urea: Often called BUN, which stands for blood urea nitrogen, urea is a byproduct of protein metabolism that’s normally filtered from the blood by the kidneys and then cleared from the body in the urine. Thus, if urea is elevated, it could very well be an indicator of kidney disease — the kidneys aren’t doing their job well enough.
A high urea concentration doesn’t always signify kidney disease, however. Sometimes, for instance, it indicates dehydration, perhaps resulting from vomiting or diarrhea. Dehydration reduces the total volume of blood, causing less blood to reach the kidneys, meaning less urea is filtered; blood levels of the substance then rise.
Lower urea levels in a chemistry profile can be caused by significant liver disease. The liver is where urea is made before heading to the bloodstream. Urea concentrations can also be low if a dog has a disease that makes her produce large amounts of urine, such as Cushing’s disease. The urea literally gets washed out.
Creatinine: Like urea, creatinine is a product of protein breakdown, and the two are always looked at “in conjunction,” says Dr. Ross, “as both are indications of kidney function.” If there’s too much creatinine in the blood, it means the kidneys are not efficiently filtering it from the body to be flushed out in the urine.
Phosphorus: Phosphorus is one of the body’s electrolytes, along with calcium, magnesium, sodium, and potassium. And like all electrolytes, “it has a wide variety of functions in the body,” Dr. Berg says, including allowing for muscle contractions and nerve function.
Phosphorus becomes elevated with a variety of disease states, “probably the most common of which is kidney disease,” Dr. Ross says, “because there just isn’t enough of the functional kidney left to excrete phosphorus from the body properly.”
Perhaps ironically, phosphorus tends to fall too low not because of a disease but because of treatment of certain diseases, Dr. Ross explains. “It’s something we monitor when we treat a dog for kidney disease or severe diabetes,” she says.
Calcium: The most common cause of elevated calcium is a paraneoplastic syndrome. Paraneoplastic syndromes are symptoms of cancer that don’t result from the physical presence of the cancer itself, but from secretion of a protein by the cancer that is similar to one of the body’s normal hormones. This results in clinical signs in body sites distant from the tumor. Elevated calcium levels are often caused by cancers that secrete a hormone that is similar to parathyroid hormone, too much of which elevates calcium levels into an abnormal range.
Far and away the most common type of cancer diagnosed in dogs with elevated calcium concentrations is lymphoma. After that comes cancer of the anal sac, and mammary gland cancers are a distant third.
A second cause of high calcium is hyperparathyroidism, in which the parathyroid glands, located next to the thyroid glands, secrete too much parathyroid hormone. This causes symptoms ranging from weakness to gastrointestinal upset and kidney damage. Fortunately, it can be treated with surgical removal of the abnormal gland.
Ingestion of some types of rat or mouse poison by a dog can also raise calcium levels in the blood (that’s what kills the rats and mice), as do de-icing compounds containing calcium carbonate. “If your dog walks where a de-icing compound has been spread, don’t allow him to lick his feet,” Dr. Ross advises. Bags of de-icing compounds should also be kept securely closed.
So should a tube of any ointment for people that contains vitamin D. Vitamin D makes the body absorb more calcium. Even licking a human where she or he has placed a topical ointment can cause a dog’s calcium levels to rise too high, “so you’ve got to be careful about that,” Dr. Ross says.
“Low calcium is much less common in dogs” than high calcium, Dr. Berg says, but there are some conditions that can cause it to drop. One is hypoparathyroidism, a condition in which the parathyroid glands secrete too little hormone. “This sometimes occurs after surgery for hyperparathyroidism,” notes Dr. Berg. “The level drops too far after being too elevated.”
Another cause of low calcium is ingestion of antifreeze. As the ethylene glycol in antifreeze gets metabolized, it combines with calcium to create calcium oxalate crystals that are deposited in the kidney. The calcium for the crystals comes out of the blood, which is why blood calcium falls too low.
Magnesium: Both Dr. Ross and Dr. Berg pretty much ignore the magnesium results on a chemistry profile. “I pay almost no attention to it,” Dr. Ross says. “We don’t know what to do with it when it’s high or when it’s low. I’m sure we’ll pay attention in the future,” when more is known about the ramifications of its levels being either elevated or decreased.
Total Protein: Total protein simply signifies the sum of the next two chemicals on the list, albumin and globulin. They are the two major sub-types of protein in blood serum. “Most clinicians look at the two values separately,” Dr. Ross says, “not the total.”
Albumin: Made by the liver, albumin is critical in maintaining blood volume by helping to hold enough water in the serum. If albumin levels fall too low, water from the blood leaks out into various tissues or body cavities. A dog could end up with “fluid in the chest or the abdominal cavity,” Dr. Ross says. Albumin also helps transport certain hormones and even some drugs to body tissues.
If the albumin concentration falls below normal, it can be because it’s being lost from the kidney because of kidney disease or from the intestine due to intestinal disease. Sometimes liver disease stops the dog from making enough in the first place. “In some unusual cases,” Dr. Ross points out, “dogs can lose albumin through their skin, say, with severe burns where the normal protective barrier is lost.
As for albumin levels running too high, the only common reason is dehydration. When there’s too little water in the blood, the absolute amount of albumin stays the same, but the concentration increases.
Globulins: Globulins are a combination of proteins, larger than albumin, that are important in fighting infections. They are key components of antibodies, which are made by certain lymphocytes. Thus, globulin levels will often go up when a dog is mounting a response to an infectious condition, or even a response to inflammation. A vet will look to find the source of the infection and perhaps prescribe antibiotics if it is bacterial. High globulin concentrations can also signify that a dog has cancer, most often multiple myeloma, a malignancy of the bone marrow, but also sometimes lymphoma.
“Low globulin is pretty rare,” Dr. Ross says, “but can be seen in dogs with congenital problems in which they are not able to produce enough antibodies to certain diseases.”
A/G Ratio: Veterinarians tend to zero in on albumin and globulin concentrations themselves. The ratio doesn’t usually add more information to help make a diagnosis.
Sodium, Chloride, Potassium: These electrolytes are often looked at as a group. Sodium “in particular, plays a role in insuring the proper amount of fluid in the body. Thus, “when we look at sodium,” Dr. Ross says, “we have to look at concentrations in conjunction with the dog’s state of hydration. If the sodium level is too high, it could mean there’s not enough water and the dog is dehydrated. If it’s too low, too much water is being retained by the blood.”
It’s a similar story for sodium and chloride assessed together. If they’re both high, that, too, can indicate dehydration. If they’re too low together, the likely cause is loss due to vomiting or diarrhea (which can also result in dehydration).
Most of the sodium in the body is located in the bloodstream, with a much smaller amount inside all the cells of the dog’s body. It’s the opposite with potassium. Most is located inside the cells, with the amount in the blood contributing a very small percentage to the total amount in the body. That said, the amount of potassium in the blood is “critical to normal neuromuscular function,” Dr. Ross says. “When potassium gets too high — and it doesn’t have to be very high — it can cause serious heart arrhythmias. These are abnormal heart rhythms that can be fatal. So we look at out-of-range potassium levels very closely.” Reasons for high potassium concentrations in the blood include kidney disease; an obstruction that keeps urine from leaving the body; or a rupture in the urinary tract. “Perhaps the dog got hit by a car and has a ruptured bladder. We have to move fast to repair it,” Dr. Ross says, “because it can kill the dog quickly.”
Low potassium can also cause illness, but it’s rarely fatal. It’s more about decreased muscle and nerve conduction.
tCO2 (Bicarb): A rough equivalent of the total amount of bicarbonate in the blood, this measurement “helps us determine whether the blood is too acidic or too basic,” Dr. Ross explains. “More commonly the concentration will be low, meaning the dog is acidotic,” she says. “We could see that with many severe diseases, like kidney failure.”
AGAP: Short for “anion gap,” this measurement, too, tells a vet whether a dog is too acidotic. “One instance in which we see levels go too high is antifreeze toxicity,” Dr. Ross says.
NA/K: “Na” is the chemical abbreviation for sodium, and “K,” for potassium. “As clinicians we usually don’t pay too much attention to the sodium/potassium ratio,” comments Dr. Ross, “although if it’s very low, less than 22, that can indicate Addison’s disease. Mostly, though, we just look at sodium and potassium values individually.”
Total Bilirubin, Direct Bilirubin, Indirect Bilirubin: Bilirubin is a product of the normal breakdown of red blood cells when they get old, or, more specifically, breakdown of the iron-carrying hemoglobin molecules in red blood cells. The body must get rid of the bilirubin, which passes through the liver and is then excreted mostly in the stools. If bilirubin is not removed, a dog will become jaundiced, with telltale yellow coloring in the mucous membranes of the gums and other areas. Complications include vomiting, fever, and weight loss.
Direct bilirubin is bilirubin that has already been metabolized by the liver. Indirect bilirubin means the liver hasn’t gone to work on it yet. “But nowadays,” says Dr. Ross, “we don’t use that distinction as much. We look primarily at the total.”
What a high bilirubin concentration means is that either too much bilirubin is being made (this can occur in autoimmune hemolytic anemia, in which the body destroys its own red blood cells), or bilirubin is not being excreted quickly enough. Inadequate excretion may indicate liver disease or an obstruction in the bile duct, perhaps caused by cancer or inflammation associated with pancreatitis. The bile duct transports bilirubin (in the form of bile) to the intestine to be excreted from the body, but if the nearby pancreas is severely inflamed and swollen, bile flow may be impaired The vet can figure out the cause of increased bilirubin in part through a simple blood test to rule out autoimmune hemolytic anemia and an abdominal ultrasound to rule out bile duct obstruction.
Alkaline Phosphatase, GGT, ALT, AST: These four chemicals, all liver enzymes, “are things we look at for liver damage,” Dr. Ross says. If the liver cells are diseased, “the enzymes leak out into the bloodstream, causing elevations,” adds Dr. Berg. “The specific elevations may help us figure out what kind of liver disease” is occurring, he notes. For instance, a high concentration of ALT, alamine aminotransferase, might indicate toxicity from a poison.
Creatine Kinase: Creatine kinase, whose shorthand is CK, comes from muscle, and if it’s elevated, it can indicate muscle damage. “But it has to be really elevated,” says Dr. Ross. “I wouldn’t even pay attention to a minor elevation. That could simply mean a needle is going through muscle to draw blood. But if it is greatly elevated, I’d consider the possibility of true muscle damage in the form of severe trauma or a disease of the muscles themselves.”
Cholesterol: “Cholesterol doesn’t cause heart disease in dogs unless the concentration is really high,” Dr. Ross says. “We are more apt to see high cholesterol in dogs with certain types of kidney disease and certain hormone-related conditions like Cushing’s disease and hypothyroidism.”
If cholesterol levels are too low, it could signify a very low-fat diet, some types of liver disease, or certain kinds of intestinal disease that don’t allow the body to properly absorb digested food properly.
Triglycerides: “Just as with cholesterol, we rarely agonize about high triglycerides in dogs,” Dr. Berg says. That said, triglycerides (as well as cholesterol) can increase with diabetes and again, with hormone-mediated conditions.
It should be noted that certain breeds, like miniature schnauzers, are prone to a genetic problem that can cause high triglycerides and high cholesterol because they don’t have enough of an enzyme that helps break down those substances. “I’ve also seen it in shelties,” Dr. Ross comments.
Amylase: An enzyme made by the pancreas, amylase is important in the digestion of food. If blood levels are too high, it could mean the pancreas has been damaged, for example by pancreatitis, and the enzyme has leaked into the bloodstream.
Osmolality: This is “another way of getting at whether a dog is dehydrated or overhydrated,” Dr. Ross says. It takes into account the concentration of many chemical particles in the blood all at once.