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Changes in white blood cell count and disease outcome

Burton AG, Harris LA, et al.  Degenerative left shift as a prognostic tool in cats.  J Vet Intern Med. 2014;28 (3):912-7.

The term ‘degenerative left shift’ (DLS) refers to findings on a complete blood count (CBC) wherein immature precursors of neutrophils outnumber mature neutrophils, implying that due to tissue demand, the bone marrow must release the immature cells prematurely because its stores of mature neutrophils have been depleted.  Diseases and syndromes involving severe infection, inflammation, or both are usually the types of conditions in which a DLS may arise.  In some cases the DLS may occur with a neutrophilia, while in others, DLS is found with a concurrent neutropenia, or a neutrophil count within the reference range.

Both cats and dogs have large storage pools of neutrophils. The only study published to date, performed in dogs, demonstrated that hospitalized dogs with a DLS were twice as likely to die or be euthanized as control dogs with the same disease diagnosis.  This retrospective case-control study of 108 cats with DLS and 322 control cats without DLS presented to a veterinary teaching hospital over a 15-year period (1995-2010) was designed to see if similar outcomes were to be found in felines.  All 108 cases had a CBC performed within 24 hours of presentation to the teaching hospital.  The 322 controls were matched by year of presentation and primary diagnosis.  Cats presented with a variety of diseases and conditions, including pyothorax, septic peritonitis, FeLV, wounds, pancreatitis, pyelonephritis, pneumonia, gastroenteritis, pyometra, FIV, and FIP, as well as several others.  Males slightly outnumbered females; the vast majority of patients were spayed or neutered. Both case and control cats were pretty evenly distributed among age groups; the age range was 0.4 to 21.2 years. One-third of the total number of patients had received previous treatment prior to presentation.

Based on hazard analysis of the data, these researchers determined that hospitalized cats with a DLS are 1.57 times more likely to die or be euthanized than cats without this finding, after adjusting for the effect of age, sex, spay/neuter status, treatment, breed, severity of left shift, and degree of neutrophil toxicity.  Cats infected with FeLV had a more than 3-fold higher hazard of death or euthanasia than FeLV negative cats.  The magnitude of the DLS also had a significant influence on the outcome; the smaller the N/I ratio (ratio of mature neutrophils to immature precursors), the higher the hazard of death or euthanasia.  Purebred and purebred-cross cats had a 1.3-fold lower risk of death or euthanasia than mixed breed groups, which may reflect an attitudinal impact on decision to treat by owners of purebred cats.

As this study spanned 15 years, there were a number of technicians and clinical pathologists evaluating the blood smears, so there is bound to be some inter-observer variability in identfying the various cell types.  Band and segmented neutrophils are sometimes difficult to distinguish clearly.  Outcomes of the study were based only on CBC findings obtained within the first 24 hours of patient presentation, so no longitudinal information regarding how the CBCs or prognosis may have changed with treatment was included.  While clinicians and owners need to be aware of the potentially poorer prognosis of a feline patient with a DLS, and the possibility of significantly increased treatment time, effort, and expense involved in such cases, each situation needs to be evaluated individually, as some patients with DLS can and will recover. Based on the limited availability of published studies of this sort, it appears that cats with a DLS may actually have a better chance of survival with hospitalization and treatment than hospitalized dogs with the same finding. [PJS]

See also:
Schnelle AN, Barger AM. Neutropenia in dogs and cats: causes and consequences. Vet Clin North Am Small Anim Pract 2012; 42:111-22.

Hostetter SJ.  Neutrophil function in small animals.  Vet Clin North Am Small Anim Pract 2012; 42: 157-71.