Numbers Without Context Are Just Numbers
A veterinary blood panel returns dozens of values, each with a reference range printed beside it. Values inside the range get a reassuring “normal.” Values outside get flagged. This binary framing misses the most important clinical question: what is the trend?
A BUN of 30 mg/dL is within normal range for most laboratories. But if that same dog’s BUN was 15 mg/dL two years ago and 22 mg/dL last year, the trajectory signals early kidney function change long before the number crosses the reference range threshold. This is why serial bloodwork matters, and why starting baseline testing before a dog is clinically sick provides the context that makes later results meaningful.
The AAHA Senior Care Guidelines (2023) and Willems et al. (2019) both emphasize that wellness bloodwork in healthy dogs catches subclinical disease earlier and at lower cost than reactive testing after symptoms develop.
Complete Blood Count (CBC): The Cellular Picture
The CBC measures the cells circulating in the blood. For aging dogs, the most clinically relevant values are:
Red Blood Cells (RBC), Hematocrit (HCT), and Hemoglobin (Hgb)
These three values measure oxygen-carrying capacity. They should be interpreted together.
What declining values mean:
- Gradual decline may indicate chronic disease anemia, a non-specific finding common in dogs with chronic inflammation, kidney disease, cancer, or chronic infection
- Acute drops suggest bleeding (potentially internal, including hemangiosarcoma rupture) or immune-mediated red cell destruction (IMHA)
- In senior dogs, a slowly falling hematocrit warrants investigation for the source of chronic blood loss or suppressed production
What elevated values mean:
- Dehydration (most common cause of a falsely elevated HCT)
- Polycythemia (rare, but important to identify)
White Blood Cell Count (WBC) and Differential
The total white cell count and the proportions of neutrophils, lymphocytes, monocytes, eosinophils, and basophils provide information about immune activity.
Elevated neutrophils (neutrophilia): suggest active infection, inflammation, or stress response. In senior dogs, persistent neutrophilia without obvious cause warrants investigation for occult infection or neoplasia.
Elevated eosinophils (eosinophilia): suggest allergic or parasitic processes. In older dogs, eosinophilia may also indicate food allergy, skin allergies, or less commonly, eosinophilic neoplasia.
Low lymphocytes (lymphopenia): commonly caused by stress or corticosteroid use. Persistent lymphopenia in a senior dog not on steroids may warrant further evaluation.
Platelet Count
Platelets are essential for blood clotting. Low platelets (thrombocytopenia) in senior dogs may indicate:
- Immune-mediated thrombocytopenia
- Tick-borne diseases (ehrlichiosis, anaplasmosis)
- Disseminated intravascular coagulation (DIC), which can accompany hemangiosarcoma and other cancers
- Bone marrow disease
Chemistry Panel: Organ Function in Numbers
The chemistry panel assesses how the major organs are performing. In aging dogs, the kidney and liver values receive the most attention, but electrolytes, glucose, and protein levels all contribute to the clinical picture.
Kidney Values: BUN, Creatinine, and SDMA
Blood urea nitrogen (BUN): a waste product filtered by the kidneys. Elevated BUN can indicate kidney dysfunction, dehydration, high-protein diet, or gastrointestinal bleeding. BUN alone is not a reliable kidney disease marker because it is influenced by many non-renal factors.
Creatinine: a more specific kidney marker than BUN, generated by muscle metabolism. The limitation: creatinine does not rise above the reference range until approximately 75% of kidney function is lost. This means a “normal” creatinine in a dog losing kidney function early is falsely reassuring.
SDMA (symmetric dimethylarginine): a newer kidney biomarker validated by Nabity et al. (2015) that detects kidney function loss earlier than creatinine, often when only 25 to 40% of function is compromised. SDMA is less affected by muscle mass than creatinine, making it particularly valuable in senior dogs losing muscle (where creatinine may appear artificially low despite declining kidney function).
The IRIS (International Renal Interest Society) staging system for chronic kidney disease uses creatinine and SDMA together, along with proteinuria and blood pressure, to classify disease stage and guide treatment.
Liver Values: ALT, ALP, GGT, and Bilirubin
ALT (alanine aminotransferase): the most liver-specific enzyme in dogs. Elevation indicates hepatocellular damage (liver cell injury or death). Even mild persistent elevations warrant monitoring, as they may represent early liver disease, drug reactions, or hepatitis.
ALP (alkaline phosphatase): elevations in senior dogs are common and often non-specific. ALP rises with Cushing’s disease (hyperadrenocorticism), steroid administration, bone disease, and biliary obstruction. In senior dogs, the most frequent cause of moderately elevated ALP is benign nodular hyperplasia (non-threatening liver nodules) or steroid-induced hepatopathy. Context and trends determine whether investigation is needed.
GGT (gamma-glutamyl transferase): primarily a biliary marker. Elevation alongside ALP suggests biliary tract involvement rather than isolated hepatocellular disease.
Total bilirubin: elevated bilirubin (jaundice/icterus) indicates liver dysfunction, biliary obstruction, or excessive red blood cell destruction. Even mildly elevated bilirubin in a senior dog warrants prompt evaluation.
Glucose
Elevated glucose (hyperglycemia): in a fasting sample, this raises concern for diabetes mellitus. Stress hyperglycemia is common in anxious dogs at the veterinary clinic, so a single elevated glucose requires confirmation with fructosamine (a longer-term glucose average marker).
Low glucose (hypoglycemia): less common but potentially serious. In senior dogs, hypoglycemia may indicate insulin-producing tumors (insulinoma), liver failure, or sepsis.
Total Protein and Albumin
Low albumin (hypoalbuminemia): can indicate protein-losing enteropathy, protein-losing nephropathy, or liver failure (the liver produces albumin). In senior dogs with chronic diarrhea and weight loss, low albumin points toward the gastrointestinal tract. With concurrent proteinuria, it points toward the kidneys.
Elevated globulins: may indicate chronic inflammation, infection, or immune stimulation. A high globulin with low albumin in a senior dog is a pattern that warrants further investigation.
Electrolytes: Sodium, Potassium, Calcium, Phosphorus
Elevated calcium (hypercalcemia): one of the “cancer clue” values. Approximately 10 to 20% of dogs with lymphoma present with hypercalcemia, and anal sac adenocarcinoma is another common cause. In a senior dog with unexplained hypercalcemia, cancer should be on the differential list.
Low sodium and high potassium: the classic pattern for Addison’s disease (hypoadrenocorticism), which can present insidiously in middle-aged and older dogs.
Elevated phosphorus with normal or declining kidney values: early sign of kidney disease progression, as the kidneys lose the ability to excrete phosphorus before creatinine rises above reference range.
Thyroid Panel
Total T4 (thyroxine): the first-line thyroid screening test. Low total T4 raises suspicion for hypothyroidism, the most common endocrine disorder in dogs. However, non-thyroidal illness (any concurrent disease) can suppress total T4, producing a falsely low result.
Free T4 by equilibrium dialysis: a more specific test that is less affected by non-thyroidal illness. Recommended when total T4 is low but clinical signs are ambiguous.
TSH (thyroid-stimulating hormone): elevated TSH with low T4 strongly supports a hypothyroidism diagnosis.
Hypothyroidism is common in middle-aged and older dogs, particularly breeds like Golden Retrievers, Labrador Retrievers, Doberman Pinschers, and Beagles. Symptoms (lethargy, weight gain, skin changes) are gradual and often attributed to aging rather than thyroid dysfunction.
Urinalysis: The Overlooked Test
Urinalysis is frequently skipped but provides information that bloodwork alone cannot:
Urine specific gravity (USG): measures concentration. Dilute urine (USG below 1.030 in dogs) in the setting of normal hydration suggests the kidneys are losing concentrating ability, an early sign of kidney disease that may precede creatinine elevation by months.
Urine protein-to-creatinine ratio (UPC): quantifies proteinuria. Elevated UPC indicates glomerular damage and is one of the IRIS staging parameters for chronic kidney disease. Persistent proteinuria predicts disease progression and warrants treatment intervention.
Sediment analysis: detects red blood cells, white blood cells, bacteria, crystals, and casts that provide information about urinary tract infections, bladder stones, and renal tubular damage.
Trending: The Real Value of Serial Testing
A single blood panel provides a snapshot. Serial panels provide a story. The most valuable patterns to track in aging dogs:
- Creatinine and SDMA trending upward (even within normal range): early kidney function loss
- ALT persistently mildly elevated (e.g., 20 to 30% above reference): chronic low-grade liver process warranting investigation
- Hematocrit gradually declining: chronic disease process or occult blood loss
- ALP rising progressively: consider Cushing’s disease screening
- USG declining over serial measurements: kidney concentrating ability diminishing
Request copies of every blood panel and keep them organized. Many veterinary practices offer client portals, but a personal spreadsheet tracking key values over time provides the clearest view of organ function trajectories.
Frequently Asked Questions
How often should senior dogs have bloodwork? The AAHA Senior Care Guidelines recommend comprehensive bloodwork (CBC, chemistry panel, thyroid, urinalysis) every 6 to 12 months for senior dogs (age 7+, adjusted by size). More frequent testing may be warranted for dogs with known chronic conditions.
What does a “normal” blood panel actually tell me? That the measured values fall within the laboratory’s reference range at that moment. It does not guarantee health. Subclinical disease often produces values that are technically normal but trending in a concerning direction. This is why serial testing and trend analysis are essential.
Should I worry if one value is slightly outside the reference range? Not necessarily. A single mildly elevated or decreased value may reflect stress, recent meals, dehydration, or laboratory variation. The clinical significance depends on the degree of abnormality, whether the finding is persistent across multiple tests, and whether it correlates with clinical signs.
What is SDMA and why should I ask about it? SDMA is a kidney biomarker that detects function loss earlier than creatinine, sometimes by months to years. It is particularly valuable in senior dogs because creatinine can appear falsely low in dogs losing muscle mass. Most commercial veterinary laboratories now include SDMA in standard chemistry panels.
Can bloodwork detect cancer? Bloodwork cannot diagnose cancer directly, but it can reveal patterns associated with malignancy: unexplained hypercalcemia (lymphoma, anal sac carcinoma), persistent anemia (chronic disease, bleeding tumors), elevated liver enzymes (hepatic masses), and low platelets (splenic hemangiosarcoma). These findings prompt further imaging and diagnostics.
Why is urinalysis important if kidney values are normal on bloodwork? Kidney disease often produces changes in urine concentration and protein content before blood creatinine and BUN rise above normal. Urinalysis catches early kidney disease that bloodwork alone would miss, sometimes by months to years before clinical signs develop.
The Bottom Line
Bloodwork in aging dogs is most valuable when interpreted as a longitudinal dataset rather than isolated snapshots. Baseline testing in healthy dogs establishes the reference point that makes later changes detectable. Trending key values (creatinine, SDMA, ALT, ALP, hematocrit, urine specific gravity) catches organ function decline when intervention is earliest and most effective. Ask for copies of every panel, track the numbers over time, and discuss trends, not just individual values, with your veterinarian.
References
- Willems A et al. Standardized wellness testing in dogs: a systematic review (Journal of Veterinary Internal Medicine, 2019).
- IRIS Staging of CKD in Dogs (International Renal Interest Society, 2023).
- Nabity MB et al. Symmetric dimethylarginine assay validation and evaluation in dogs with chronic kidney disease (Journal of Veterinary Internal Medicine, 2015).
- AAHA Senior Care Guidelines for Dogs and Cats (AAHA, 2023).