Age-related changes in canine and feline clinical pathology, and how to interpret them

We see many differences between blood profiles of young, growing dogs and cats and their adult counterparts, which may reflect growth or immaturity. There are also changes which are recognised as being due to ageing in more elderly patients. Various studies have looked at haematology and chemistry differences between young/growing dogs and cats compared to reference intervals established for adults, although some of these only looked at specific breeds, and they often look at slightly different age brackets. This is a summary of the most common and significant changes you may see.  

Haematology

In terms of haematology, one of the most notable features is in the red blood cell (RBC) counts. In dogs, these values start off near adult levels but decline over the first eight weeks due to the destruction of foetal RBCs and expansion of blood volume relating to growth without sufficient accompanying erythropoiesis to keep up. There may also be a degree of iron deficiency at weaning. Values steadily increase and approach adult reference interval (RI) from four months of age, but this can take 6 to 12 months. In kittens, adult values are usually reached by the age of four months. Both puppies and kittens also have increased mean corpuscular volume (MCV), particularly in the first month of life.

To compensate for the low RBC values, puppies and kittens have reticulocytosis, reflecting accelerated erythropoiesis

To compensate for the low RBC values, puppies and kittens have reticulocytosis, reflecting accelerated erythropoiesis. They have significantly higher counts than adults up to around eight weeks of age, which then fall to within adult ranges by five to six months old.

The combination of red cell values below the adult RI and reticulocytosis could be misinterpreted as a regenerative anaemia and trigger further investigation, particularly when neonates may present with non-specific signs of illness.

This highlights the importance of knowing the age-related differences to expect, correlating with clinical signs and potentially monitoring if there is concern regarding a genuine anaemia (eg if the patient had a significant flea or worm burden).

Lymphocyte counts are generally significantly higher in dogs and cats less than six months old, due to immune stimulation as they encounter different antigens

Lymphocyte counts are generally significantly higher in dogs and cats less than six months old, due to immune stimulation as they encounter different antigens. Adrenaline response associated with excitement can also contribute to this, particularly in cats. Lymphocytosis is not documented in all canine studies but where it is, counts are generally significantly higher than adult RIs. 

Biochemistry

We also see biochemical changes in young dogs and cats. The parameters which are most commonly significantly elevated are primarily associated with bone growth. These changes generally persist until the individual is fully grown and may therefore be more dramatic in larger breeds.

You may remember that there are multiple alkaline phosphatase (ALKP) isoenzymes, including bone ALKP from osteoblasts. This accounts for the significant increases in ALKP above adult RIs, often two or three times the upper reference limit. Levels are particularly high in the days immediately following colostrum ingestion, which may reflect colostral ALKP or ingestion of colostrum stimulating increased production.

We also see biochemical changes in young dogs and cats. The parameters which are most commonly significantly elevated are primarily associated with bone growth

Levels of phosphorus and calcium are also increased due to bone growth and remodelling as well as the effects of growth hormones on renal reabsorption. In adults, high calcium-phosphorus product leads to metastatic mineralisation of soft tissues, but this is not seen in immature animals.

Creatine kinase (CK) is also commonly significantly increased in growing animals, and although birth trauma could contribute to this in the early days, it is not clear why this persists.

The parameters which are most commonly lower than adult values are proteins – total protein, albumin and particularly globulins – reflecting the immaturity of the immune system. If you sample neonates, you may see higher globulin levels initially due to colostrum absorption, which is metabolised over the first month or so of life. Then, the animal’s own level is low and steadily increases as the individual is exposed to antigens, reaching the adult RI at around six months old. The difference in albumin is milder and may reflect lower hepatic function and intestinal absorption.

Creatinine may also be below adult levels, probably due to lower muscle mass.

What about in elderly patients?

We can also see changes in routine blood test results at the other end of the spectrum, in elderly patients. In humans, anaemia of the elderly is well described and is multifactorial with causes including iron deficiency, chronic disease, CKD and myelodysplasia.

A study looking at clinically healthy aged dogs found changes in laboratory values which support iron-restricted erythropoiesis, inflammation and potentially GI bleeding, which suggest a similar trend to that seen in elderly humans (Radakovich et al., 2017).

We commonly see raised liver enzymes, particularly ALKP, in older dogs, due to benign nodular regenerative hyperplasia

In the same study, increases were seen in total protein and globulins with a reduction in albumin, which could all reflect low-grade inflammation. Urea was increased without concurrent creatinine elevation, which can be noted with dehydration, but gastrointestinal haemorrhage is also possible. We commonly see raised liver enzymes, particularly ALKP, in older dogs, due to benign nodular regenerative hyperplasia.

Subsequent work in this area of geriatrics will be invaluable to help further our understanding of ageing and its impact on clinical pathology.