Dr Elizabeth Youens BSc(Hons) BVSc MRCVS
When presented with a sick cat, many veterinarians will reach for a complete blood count (CBC) as one of their primary diagnostic tools and as part of a minimum database. A feline CBC can reveal many things: evidence of dehydration, infection, inflammatory markers, the presence or absence of anemia, and more.
When presented with a sick kitten, perhaps only a few weeks of age, you may assume to do a CBC after assessing a complete history and clinical signs, as well, alongside, perhaps, a chemistry profile. However, can you apply the CBC and chemistry interpretation framework you have for adult animals to kittens as well?
As neonatal kittens mature and reach adulthood, their physiology changes. Due to this, feline CBCs require age-specific interpretation to provide accurate evaluation, and therefore the most appropriate care.
Veterinarians use objective animal diagnostics such as a CBC or biochemistry to provide measurable and specific parameters to interpret alongside a patient’s medical history and clinical exam. Hematology is an important diagnostic tool, and can reveal vital insights into potential pathologies, such as infectious diseases or autoimmune disorders.
The results must always be interpreted in the context of the individual, as there are age, sex, and breed differences in cat hematology. Unfortunately, not all of this is well established by animal reference laboratories or peer-reviewed research. Values produced in a CBC may fluctuate within a certain range, with some parameters more closely regulated than others. Healthy neonates, from the first few days of life and well over 12 weeks of age, may produce hematological values outside of the reference range for adult animals, leading to confusion and even mis-diagnosis.
The sample requirement for a complete blood count is usually a small amount of venous blood. Venipuncture in cats and kittens is described in various locations. Depending on the health status and stability of the patient, one method will be more appropriate than others. In stable patients, a blood draw is usually performed using the jugular vein, with the pet in dorsal recumbency, and head and neck extended. Aspiration of blood should be slow to avoid collapsing the vein, place it into an EDTA, and slowly invert to prevent clotting.
At Moichor only 0.25mL ml of whole blood volume is required to perform basic testing. If a blood chemistry were required, an additional 0.25mL in a serum separator would be the minimum requirement. In unstable patients, a peripheral vein would be preferred as these sites are ideal especially when coagulation ability is in question.
Often, diagnostic test results are interpreted by use of reference intervals, which are provided by research published in peer-reviewed journals and passed on to the reference laboratory or veterinary pathologist. Individual test results are then considered ‘normal’ if they fall within this reference range.
These hematology and chemistry reference ranges are therefore essential for accurate interpretation of results. The reference intervals are usually based upon a closed colony of adult cats, with little variation in age or breed. As we know that kitten physiology differs from that of an adult cat, the standardized adult reference interval may therefore not be appropriate when interpreting young kitten chemistry and hematology. Interpretation of results must be performed with care in neonatal kittens, with a focus on analyzing results with respect to the individual.
Nakai et al., (1992) analyzed both hematological and biochemical values in 91 cats and found many age-related changes to physiological variables. Red blood cells and hematocrit and hemoglobin concentrations were all initially low before stabilizing by 9-11 months of age. White blood cell counts started high in neonatal kittens and then decreased to a plateau.
Moon et al. (2001) and von Dehn (2014) reviewed the available hematological data for cats and its variations for kittens and provide practical advice for management of pediatric cases.
According to both Moon et al. (2001) and Nakai et al. (1992), there are some significant differences between kitten and adult cat physiology when it comes to hematology values. Here are some figures adapted from Moon et al. (2001) to demonstrate some key differences.
If you’re not lucky enough to have a veterinary pathologist interpretation of every CBC, it’s important to have some practical knowledge about how to analyze results to provide best care to all age ranges of feline patients. When interpreting a kitten CBC, it is important to recognize that some outlier values may be due to age, rather than a disease process, and to know whether a variation from an adult reference interval is due to age-related differences of healthy neonates or because of genuinely sick neonates.
Here are some key takeaways from the available data.
· PCV is around 35% in neonates then reduces to between 25-30% between 2-4 weeks and then returns to approximate adult values (around 35%) by 16 weeks.
· It is normal to see low RBC parameters in young kittens. Values do not normalize to adult levels until around 4 months old.
· WBC counts can vary widely as kittens grow and develop. They can start high at birth, then decrease before stabilizing. Care must be taken not to over-interpret values outside of reference range in young age groups.
· PCV and MCHC values reach adult levels by around 4 months of age.
· RBC and hematocrit levels don’t stabilize until around 10 months of age.
· WBC parameters can still vary up until around 18 months, so it is not just very young kittens that can have affected results.
Here are some practical examples of scenarios which demonstrate the need for accurate assessment of standardized reference ranges:
· A PCV of 25% in a neonate could lead to a young kitten being misdiagnosed with anemia, when that is in fact a normal value.
· A PCV of 35% in a young kitten being thought of as normal when it is in fact high due to dehydration. Fluid status is hard to assess in neonates, so this is a particular problem to be aware of.
· A WBC count of 20.1 ×106/μL being assessed as leucocytosis, leading to investigation of infectious, inflammatory or neoplastic causes, when in fact it is normal for the age.
· Equally, a low WBC being assessed as normal, when the kitten is actually leucopenic for its age, due to, for example, acute viral disease, septic shock or toxemia.
A medical history, the clinical signs and data based on a physical exam, such as hydration status, rectal temperature, heart rate, body weight and respiratory rate, will obviously play a large role in diagnosis of disease. However, objective measures such as blood testing provide vital information - providing they are used correctly. Considering age, especially in neonatal kittens, when interpreting critical tests such as feline CBCs will improve the accuracy of a practitioner's conclusions on a patient's health status.
Moon P, Massat B, Pascoe P. (2001) ‘Neonatal critical care.’ Vet Clin North Am Small Anim Pract 31:343-367
Nakai, N., Nawa, K., Maekawa, M. & Nagasawa, H. (1992) ‘Age-related changes in hematological and serum biochemical values in cats.’ Jikken Dobutsu 41:3 pp.287-94
Von Dehn, B. (2014) ‘Pediatric clinical pathology’ Vet Clin North Am Small Anim Pract 44:2 pp. 205-219