March 2022

In this VETgirl online veterinary continuing education blog, Dr. Justine Lee, DACVECC, DABT reviews the importance of the BIG 4 (e.g., PCV/TS/BG/BUN) in every day veterinary medicine. The BIG 4, often called Quick Assessment Tests (QATS) or Minimum Database (MBD), allows for rapid assessment of the veterinary patient. While some clinicians often “blow” this off, you can gain a WEALTH of information from these 4 tests.

As an emergency critical care specialist, I utilized the BIG 4 EVERY. SINGLE. DAY in the veterinary ER, as the PCV/TS gives you so much information! Read on to learn more!

By Dr. Justine Lee, DACVECC, DABT
Director of Medicine / CEO, VETgirl

The Importance of the BIG 4 PCV/TS/BG/BUN in veterinary medicine

Just sent your blood work to an outside lab? Not going to get the blood results back until tomorrow? Just got the blood work a day later, and too embarrassed to call your owner to tell them that their pet is in severe kidney failure or is a diabetic ketoacidotic (DKA) patient and you didn’t pick it up until 24 hours later? When in doubt, check a BIG 4 on your patient before sending out that blood. This simple two-minute quick assessment test will benefit your clinic in multiple ways: not only will it increase patient care and help you rapidly recognize just how sick your patient is, but it will help you fine tune differential diagnoses for your owner and potentially increase your revenue. Quite simply, with these four rapid tests you can rule out kidney failure, diabetes mellitus or DKA, anemia, severe dehydration, severe hypovolemia, blood loss, or diseases such as protein-losing enteropathy (PLE) or nephropathy (PLN) within seconds.

Next, any patient that you have hospitalized on IV fluids should have a BIG 4 performed daily, as it will help guide further treatment and fluid therapy. One can use PCV/TS as a tool in evaluating hydration status on a daily basis. Normal healthy patients on IV fluids should be hemodilute and have a PCV of 30-35% and a total solids (TS) of 5-5.5 mg/dL. Likewise, a normal, healthy patient on IV fluids should have a specific gravity between 1.015-1.018 to indicate appropriate hydration. Unfortunately, certain underlying diseases affect specific gravity (including diabetes mellitus, hyperthyroidism, hyperadrenocorticism, renal failure, furosemide administration, etc) and prevent us from being able to accurately assess hydration with specific gravity alone.

PACKED CELL VOLUME AND TOTAL SOLIDS
In the chart below, evaluation of PCV/TS on initial presentation (pre-fluid therapy) can help fine tune your differential diagnoses.

Another benefit of the PCV/TS is the assessment of the color of the serum in the hematocrit tube. Any indicator of icterus, hemolysis, or lipemia will help fine tune your differential diagnoses and aid in further diagnostics. For example, an anorexic cat that is febrile, vomiting, and has icteric serum on initial blood work would suggest underlying liver disease (such as hepatic lipidosis, cholangiohepatitis, or the triad of pancreatitis). A hemolyzed blood sample in a severely anemic dog would suggest immune-mediated hemolytic anemia (IMHA), and warrants doing an immediate slide agglutination test (see proceedings for “Emergency Management of the IMHA Patient”). A severely lipemic sample in a vomiting patient should make the clinician suspicious of pancreatitis. Likewise, in any severely hyperglycemic patient, the serum in the hematocrit tube can be used to evaluate for the presence of serum ketones (using a urine dipstick).

PCV/TS VETgirl

BLOOD GLUCOSE (BG)
When measuring blood glucose as part of the BIG 4, one will detect either euglycemia, hypoglycemic, or hyperglycemia. Generally, a handheld glucometer is accurate in the middle to high range, but often runs 10-15 mg/dL lower than normal in the low-range; this design is to stimulate a human (with diabetes) to eat to prevent hypoglycemic shock.

Who should we decide to check a BG on? When in doubt, a BG should be checked in any patient showing generalized malaise. It is also imperative that a BG be checked in all neonatal or pediatric patients who present for weakness, vomiting, and diarrhea to ensure that they are not hypoglycemic. That said, any BG < 55-60 (depending on clinical signs) should be appropriately treated with an immediate meal (if there are no contraindications such as vomiting or risks of aspiration) or with IV supplementation of 50% dextrose (0.5-1.5 ml/kg, diluted 1:2 or 1:3 with IV saline, given over 1 minute), followed by a constant rate infusion (CRI) of 2.5% to 5% dextrose supplementation in maintenance fluids. Adult patients should never become hypoglycemic from malnutrition alone, and if detected, ruleouts should include lab error, insulin overdose, benign or malignant liver tumors (hepatomas), insulinomas, or sepsis. Evidence of hyperglycemia, on the other hand, should prompt further diagnostic evaluation for the following differentials: stress hyperglycemia (cats), diabetes, inappropriate dextrose supplementation, and hypovolemia. In patients with a severe hyperglycemia (> 200 mg/dL), rule outs such as diabetes mellitus should be considered. A thorough history should be performed to query the owner on presence of clinical signs such as polyuria, polydypsia, weight loss, etc. The presence of glucosuria and repeatable documentation of hyperglycemia or glucosuria should warrant a workup of diabetes mellitus. When presented with a mildly hyperglycemic canine patient (i.e., BG 159 mg/dL), one must carefully assess the patient for hypovolemia, shock, or poor perfusion. The presence of a mild hyperglycemia in a non-diabetic, canine patient (with no history of pu/pd, weight loss, glucosuria, etc.) is highly suggestive of a “stress of death” (SOD); in other words, the patient is severely hypovolemic, and as a result, is releasing massive stress hormones (i.e., cortisol, epinephrine, etc.) resulting in this mild hyperglycemia. This patient should be aggressively stabilized and volume resuscitated if appropriate.

AZOSTICK (BUN)
Lastly, one can measure preliminary renal function with an AZOSTICK (which is a gross estimate of BUN). The presence of a mild azotemia may be either pre-renal or renal disease, and ideally, a urine specific gravity (sp. gr.) should be obtained prior to starting IV fluids to differentiate the two. A BUN > 50-80 (highest reading) should immediately prompt further blood work (such as a venous blood gas measuring creatinine or BUN, or a more specific measurement of creatinine). As long as there are no contraindications for cystocentesis (such as pyometra, coagulopathy, ascites, thrombocytopenia or thrombocytopathia, etc.), a sp. gr. should be assessed prior to any fluid therapy to help evaluate appropriate urine concentration. Keep in mind that underlying diseases such as hyperadrenocorticism, hyperthyroidism, diabetes, psychogenic polydipsia, etc. will affect the ability to judge renal concentration.

These quick assessment tests provide you as a clinician the ability to rapidly diagnose and refine your differential diagnoses, and provide a less expensive monitoring parameter for clients. The BIG 4 can be utilized in both general and emergency practice, as it allows one to thoroughly evaluate a patient.

The short of it? Do MORE Big 4’s (or whatever you want to call it) EVERY. SINGLE. DAY in veterinary medicine, especially for those patients presenting ill or on IV fluids!

  1. We do not perform Azostick at our hospital, I am very excited to learn more about them and present it to our practice manager to try and see if we can start ordering them.

  2. Definitely makes me think more about why we’re doing these tests as a Vet Nurse. I will definitely be asking my docs to do more of this on presentation.

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