You run and review the results of complete blood counts and serum chemistry profiles all the time. But important conclusions may elude you if you rush through your analysis. Here are a few common areas you might overlook.
Have a hypothesis
Always have a hypothesis or prediction about what you will find in the laboratory data. In many ways the laboratory data should be confirmatory, not exploratory. The very best clinicians are thinking two or three steps ahead. Not only do they have a hypothesis about what they will find, they already know what they will do next based on whether or not their hypothesis was correct.
Keep your hypothesis in mind while interpreting the laboratory data. Even if your hypothesis is not true, it helps you understand what you know about your patient and you have important information to communicate to the client.
Don't forget the total protein
Many commercial veterinary laboratories don';t include a total protein (or total solids) concentration on a complete blood count (CBC), which makes it difficult to interpret the packed cell volume (PCV) in an anemic animal. I suggest that you determine a PCV and total protein concentration on every sample before you send it to the laboratory.
Run a reticulocyte count
If you follow the advice above and measure a PCV and total protein concentration before you send a sample to the laboratory, you will know to order a reticulocyte count right off the bat.
Another excellent option is reflex testing. This is an option that most commercial laboratories will set up for your hospital. Basically, any CBC you submit that has a PCV below the reference interval (or a PCV value of your choosing) will automatically trigger a reticulocyte count. This may gain you as much as two days' time working up the case. Some reference laboratories already report reticulocyte counts on every CBC.
Never trust a computer
In-hospital analyzers have come a long way since the days of the Coulter counter. But there is no substitute for an experienced technologist or pathologist reviewing your patients' blood smears. In my opinion, on a CBC, an experienced individual should always look at a blood smear in sick patients. In-hospital analyzers can easily miss important clues or even the diagnosis. They can't identify nucleated red blood cells, spherocytes, schistocytes, Babesia species, Cytauxzoon species, Ehrlichia species, bands, immature granulocytes, toxic changes, and much more.
You can train your staff to help in analysis with little effort. You likely send at least one CBC a week to a reference laboratory. Before you send out the sample, have your staff members perform their own white blood cell differential and comment on morphology. When the analysis comes back from the laboratory, make sure you are "giving them a grade" and rewarding good performances, or they won't be invested in getting better. This will be incredibly helpful when you need a sample interpretation immediately.
Get more blood than you think you need
Don't fear the jugular vein (unless a patient has a bleeding disorder). Get an extra EDTA tube and serum in case your empirical treatments don't work. We save every sample for at least seven days. I can;t tell you how many times we have gone back to get a preantibiotic sample for PCR testing or checking something that wasn't on the original laboratory work. It's better to have it and not need it than to need it and not have it.
Don't fear the flow sheet
Take full advantage of technology and critically evaluate laboratory work, especially data collected during wellness evaluations. Trends are often apparent long before your patient shows signs of illness. Decreasing renal function is frequently identified by gradual increases in creatinine concentrations or persistent isosthenuria before it exceeds the reference interval.
Look past the Hs and Ls
Unfortunately, reference intervals can't always tell the entire story. Many reference intervals are created using apparently healthy pets that can have subclinical disease. Some values tend to have wide reference intervals. Examples include serum albumin, globulin, and creatinine concentrations:
- Many albumin reference intervals include values below 3 g/dl and over 4 g/dl. Any albumin concentration below 3 g/dl raises a red flag for me and prompts a search for decreased production or loss.
- Globulin intervals are often even wider, and panhypoproteinemia is often missed because the low end of the reference interval is ≤ 2 g/dl. Any globulin concentration ≥ 3.6 g/dl might indicate an inflammatory or even a neoplastic process.
- Never ignore a high normal or mildly increased creatinine concentration since you have to lose 75% of kidney function before creatinine becomes elevated.
Troublesome reference intervals on a CBC include hematocrit/PCV and platelet counts. When I see a PCV toward the lower end of the reference interval, I think hard about decreased production, destruction, or loss of red blood cells. Some platelet count reference intervals dip below 200,000/µl. Platelet counts ≤ 200,000/µl always raise a red flag for me.
Don't forget the urinalysis
A complete urinalysis can hold a treasure trove of information. A dipstick and specific gravity are great, but sick animals should always have a sediment examination. The urinary tract is a frequent source of inflammation or infection, and the kidneys are common bystanders that can be injured in times of crisis. Examining an air-dried modified Wright's-stained urine sediment increases both sensitivity and specificity of bacterial identification.
Adam Birkenheuer, DVM, DACVIM, PhD Department of Clinical Sciences College of Veterinary Medicine North Carolina State University Raleigh, NC 27607