Is your pet feeling “off”, or even downright sick, and all the blood tests are normal? This doesn’t seem possible, right? Shouldn’t something be high or low? How can your dog or cat be sick, and none of the tests show anything? Here we discuss that situation and explain which diseases may not show up on standard blood tests.

What are Blood Tests?

“Blood Tests” may have different meaning to different veterinarians, veterinary specialists, and pet-owners. The tests that are most often being referred to include the complete blood count (CBC), comprehensive chemistry panel (chemistry), electrolyte panel and urinalysis (UA) — technically, the last is not a “blood test”, but is considered part of the so-called “minimum database” of tests: CBC, chemistry, electrolytes, and UA. These tests evaluate the major organ systems in the body, but there are some diseases that can escape notice with just these tests.

How can diseases avoid detection with standard blood tests?

Some organs and diseases do not affect the minimum database of tests. The central nervous system (CNS) — which includes the brain and the spinal cord — is a good example of this. This important organ system is good at isolating itself from the blood stream, and therefore some CNS diseases will not alter blood tests. Other examples of organs that can become diseased without affecting standard blood tests include the eyes, digestive system, and reproductive system.

Which organs, and diseases, may not show up on standard blood tests?

• Central Nervous System (CNS) — Diseases that affect the brain, spinal cord and cerebral spinal fluid (CSF) can cause symptoms including lethargy, listlessness, weakness, ataxia (wobbliness) and even seizures. Meningitis, encephalitis, and cancers of the nervous system are examples of diseases that frequently do not show up on standard blood tests. To diagnose these, specialized tests are needed, which could include advanced imaging (MRI or CT scan), spinal tap and CSF analysis.

• Musculoskeletal System — The musculoskeletal system includes muscles, bones and joints. These diseases can cause your pet to be sore, limping, holding up a paw, hobbling or even unable to stand. Arthritis (age-associated, immune-mediated polyarthritis — IMPA), broken (fractured) bones, myasthenia gravis, and bruised or injured muscle (from a fall, accident, injury) frequently do not affect standard blood tests. Diagnosing these diseases sometimes needs X-rays (radiographs), specialized blood testing, CT scan, joint fluid analysis or a muscle biopsy.

• Ears — Diseases that affect the ears can cause your pet considerable pain, anxiety, less interaction with you and the environment and reduced quality of life. Symptoms such as head shaking, scratching, dizziness, deafness, and pain can occur. Diseases include allergies, ear infections (outer, middle, and inner ear) and parasites (ear mites) which inhabit the ear canal. Ear diseases usually need to be evaluated with visualization of the ear canal with an otoscope or CT scan and bacterial culture of any secretions.

• Skin — The skin is often overlooked and not thought of as an “organ”, even though technically it is the largest organ in the body. Disorders of the skin can cause itching, insomnia, anxiety and pain. Many skin diseases do not cause changes in the standard blood tests, including skin allergies (food allergy, atopy), parasites (Demodex, Scabies, fleas, ticks), and skin cancer. To diagnose these disorders, testing such as skin sampling (biopsy, “skin scrape”) or allergy testing is typically needed.

• Eyes — Dogs and cats can develop vision problems from eye diseases which usually do not show up in standard blood tests. Symptoms of eye diseases include ocular discharge, squinting/blinking (blepharospasm), red eye, discolored cornea, mismatched pupil size and even blindness. Disorders such as cataracts, glaucoma and uveitis (inflammation of the inside of the eyeball) need specialized testing such as corneal staining, tear production assessment and intraocular pressure (IOP) measurement.

• Heart — Even though all blood in the body passes through it, heart diseases frequently do not alter the standard blood tests. Symptoms of heart diseases include weakness, unwillingness to go on walks, fainting, coughing, and difficulty breathing. Diseases such as high blood pressure (hypertension), cardiomyopathy (hypertrophic cardiomyopathy, dilated cardiomyopathy), arrhythmias such as atrial fibrillation, congestive heart failure and pericardial effusion (fluid accumulation around the heart) usually need specialized tests such as EKG, angiogram, echocardiogram and cardiac enzyme measurement.

• Respiratory — If your pet is wheezing, coughing, gagging, breathing quickly (panting), cannot tolerate walks, sneezing excessively, has nasal discharge or a bloody nose (“epistaxis”), then they may have a respiratory condition. Diseases such as pulmonary fibrosis, blood clots in the lungs, pulmonary hypertension, bronchitis, cancer (lung, nasal), viral infections, nasal polyps, nasal fungal infection (Aspergillus) are examples that require specialized testing, including X-rays (radiographs), endoscopy and CT scan imaging.

• Urinary tract — Organs include the kidneys, ureters, bladder, and urethra. Symptoms of urinary tract diseases include straining to urinate (stranguria), blood in the urine (hematuria), accidents in the house and having to urinate more frequently (pollakiuria). Urinary tract diseases that frequently do not cause abnormal blood values include bladder or kidney stones, bladder infection, urinary tract cancer, ectopic ureter, urethral sphincter mechanism incompetence (“weak valve”), benign renal hematuria, Feline Idiopathic Cystitis (FIC), and reflex dyssynergia. Typically, specialized imaging tests including ultrasound and uroendoscopy are needed to diagnose urinary tract disease.

• Digestive — Symptoms of digestive diseases can include loss of appetite, refusing food, vomiting, regurgitation, gurgling stomach, diarrhea, and weight loss. Diseases affecting the digestive system usually require specialized testing and will not show up on routine blood tests. Examples of diseases and conditions include inflammatory bowel disease (IBD, chronic enteropathy), pancreatitis, Exocrine Pancreatic Insufficiency (EPI), digestive cancer, parasites, foreign material, or esophageal strictures.

• Hepatobiliary — Many diseases of the liver and gallbladder do cause changes in the routine blood tests, however, a few hepatic disorders require specialized blood or imaging testing, including gallstones and congenital liver diseases (microvascular dysplasia, portosystemic shunt). Symptoms of hepatobiliary diseases could include pain associated with eating, behavioral changes after meals and poor growth throughout puppyhood or kittenhood.

• Reproductive — Even spayed and neutered dogs and cats can develop reproductive diseases. Symptoms can include discharge from the vulva or penis, painful or bloody urination, lethargy and fever. Examples of reproductive conditions or diseases that may not be apparent on routine blood testing include pregnancy, prostatitis, prostatic cysts, prostatic cancer, vaginitis and vaginal stricture.

• Infectious — There are many different types of infections including viral, parasitic, bacterial and fungal. Not all infections cause changes in blood tests, especially viral infections. Not every infection causes a high white blood cell (WBC) count. Symptoms of infections can include listlessness, fever, cough, swelling, discharge/drainage. Most infections require specialized testing including serology (antibody measurement), PCR or culture. Examples of just a few of infectious organisms that may not cause changes in the standard blood tests include rabies, FeLV (Feline Leukemia Virus), FIV (Feline Immunodeficiency Virus), heartworm disease, and parasites (intestinal, respiratory, urinary, CNS and musculoskeletal).

• Endocrine — There are variety of hormonal diseases which may not alter routine blood tests and that require specialized blood, urine, or imaging testing to diagnose. Examples of these include adrenal gland diseases (pheochromocytoma, Addison’s disease), diabetes insipidus and thyroid disease (hypothyroidism, hyperthyroidism). Symptoms of hormonal diseases can include excessive thirst, weight gain, weight loss, warm-seeking behavior, haircoat changes, appetite changes (reduced, or increased).

Although many internal medicine diseases will result in abnormalities in the standard blood tests, you can see that there are many that may not be noticed with just those tests and require additional testing. If your pet is sick and the standard blood tests are not revealing the cause, contact us, we can help.