In this VETgirl online veterinary continuing education blog, we review the prevalence of thrombocytopenia in cats. Before you blame it "platelet clumping," let's make sure it's not from something else!
Have you ever received blood work results on a cat and weren't quite so sure what to make of the low platelet count? It seems like a high prevalence of cats show thrombocytopenia on machine-run lab work, yet these cats either don’t exhibit any clinical bleeding tendency, or a slide review often reveals platelet clumps that skewed the machine’s measured count. Even though a good handful of these feline cases only have artifactual thrombocytopenia, what do we make of the truly thrombocytopenic cat? Typically, with thrombocytopenia in dogs and cats, we become concerned about underlying diseases such as bone marrow disease (e.g., lack of production), destruction (e.g., immune-mediated thrombocytopenia secondary to cancer or infection), and consumption (such as disseminated intravascular coagulation, thrombosis, or bleeding). But what about cats? What are the primary causes of thrombocytopenia?
Well, Ellis et al of the United Kingdom wanted to evaluate the etiologic causes for feline thrombocytopenia. They also wanted to evaluate the prevalence of true thrombocytopenia in cats, and identify the portion of thrombocytopenic cats that were infected with Feline Leukemia (FeLV) or Feline Immmunodeficiency Virus (FIV). So, they retrospectively evaluated medical records from a referral hospital in England between September 2008 to January 2017. Cases were selected that documented an automated platelet count with a confirmatory manual slide review consistent with thrombocytopenia. In this study, the authors defined thrombocytopenia as < 200 x 109/L (reference range 250-800 x109/L) and severe thrombocytopenia as <50x109/L. If platelet clumps were seen on slide review, the medical record was excluded from this study. Ellis et al evaluated each case for clinical signs related to the cat’s presenting disease, including whether or not there were signs of spontaneous hemorrhage. They reviewed available lab testing, paying close attention to the FeLV/FIV test results to evaluate for any increased incidence of these particular viral infections among cats exhibiting thrombocytopenia.
Overall, the authors identified 470 cases of feline thrombocytopenia, but over half of these cats were either missing a slide review or the provided slide review was in conflict with the automated measurements and supported normal platelet counts. Of the 2544 cats that entered the hospital during that time period receiving both hematologic testing and a slide review, only 194 cases fit the inclusion criteria for thrombocytopenia. For a control group, the investigators used an equal number of cats with documented normal platelet counts. The authors found that the median platelet count for thrombocytopenic cats was 112×109/L (range 1 to 198×109/L), and the median platelet count for the control group was 354×109/L (range 251 to 800×109/L). Ninety-three of the thrombocytopenic cats were tested for FIV and FeLV; of these, 11% were positive for FeLV and 3% were positive for FIV. In the control group, only 45 cats were tested for FeLV and FIV and only one cat tested positive for FIV with none testing positive for FeLV.
The investigators then categorized the thrombocytopenic cats in two main ways: pathomechanism for thrombocytopenia and underlying disease process. For unspecified reasons, the investigators only attempted to categorize 112 of the 194 thrombocytopenic cats by their thrombocytopenia pathomechanism. Of these 112 cats, 82 (42%) did not have an identifiable cause. Most cats were classified into “loss” (20%) or “failure of production” (18%) categories, with “immune-mediated” (11%) and “consumption” (9%) trailing behind. No cats fit the “sequestration” category. The investigators then categorized the cats by their disease process and found the following disease prevalence in descending order: hematological disease, infection, neoplasia, trauma, hepatic and/or pancreatic disease, other, disease localized to the urinary tract, gastrointestinal tract, nervous system, respiratory system, and the cardiovascular system. While categorizing the cats, the investigators noticed a subpopulation of cats that had severe thrombocytopenia (< 50x109/L). Eleven of these cats had uncategorized disease. Another 11 had bone marrow disease and 6 had immune-mediated thrombocytopenia. Four were positive for FeLV.
Only 7% of all thrombocytopenic cats showed evidence of hemorrhage from any cause at presentation. Over the timespan of this study, thrombocytopenia was found in cats with a prevalence of 5.9% (which is pretty low!). Female cats were slightly overrepresented in this study, which may be a type 1 error. Vaccination within the past 12 months was found to be associated with the occurrence of thrombocytopenia, but the authors question the clinical significance of this finding. The authors state that the 11% incidence of ITP is low, which is consistent with findings from prior feline studies. However, in dogs, the most common cause of thrombocytopenia is immune-mediated causes. It is more common for a cat to have an inflammatory or infiltrative splenic sequestration of the platelets, or a bone marrow disorder. Those that were suspected to have a primary immune-mediated thrombocytopenia had a lower platelet count than other causes of thrombocytopenia, which these authors state is a consistent finding in dogs as well.
The limitations of this study must be considered however. First, it is retrospective in origin. Second, unless VETgirl missed it while reviewing this paper, we couldn't identify how the authors defined immune-mediated thrombocytopenia (e.g., what platelet count!?). Third, the power was poor in this study, due to low numbers. For example, less than 50% of the cats in this study actually had a FeLV/FIV test run. Also, very few of these cats actually had a full coagulation panel performed (e.g., PT, PTT), so how did the authors rule out DIC? As VETgirl see's disseminated intravascular coagulation (DIC) all the time in the ICU, we want to make sure that DIC wasn't underdiagnosed in this study! Finally, without bone marrow sampling or running platelet antibody testing, it is difficult to rule out underlying disease (e.g., bone marrow suppression, neoplasia, ITP, etc.). Huge limitations, unfortunately.
So, what do we take from this VETgirl podcast? If you end up diagnosing a cat with a measured thrombocytopenia on automated analysis (e.g., every day in VETgirl's life), it should always warrant a slide review for platelet estimate before embarking down a work-up for thrombocytopenia. As this study found that the three most common disease processes associated with thrombocytopenia in cats included hematologic disease, infectious disease, and neoplasia, appropriate work up (including FeLV/FIV status, radiographs, complete blood count, ultrasound, bone marrow, coagulation panel, etc.) is warranted. While this study found that the prevalence of FeLV is decreasing in Northern Europe, this will vary geographically and still should be ruled out in ill cats.
Ellis J, Bell R, Barnes DC, et al. Prevalence and disease associations in feline thrombocytopenia: a retrospective study of 194 cases. J Small Anim Pract 2018;Jan 22:1-8. doi: 10.1111/jsap.12814. [Epub ahead of print]