Hello.
My name is Dr. Rajiv Pruthi.
I am a professor of medicine in the Mayo Clinic College of Medicine, a co-director in the
Special Coagulation Laboratory and director of the Hemophilia Center at Mayo Clinic in
Rochester, Minnesota.
I will be presenting on heparin induced thrombocytopenia, and approach diagnosis and the role of the
functional serotonin release assay.
I have no relevant disclosures for this presentation.
At the end of this presentation, the audience will understand the clinical presentation
and pathophysiology of heparin induced thrombocytopenia, also known as HIT, know the HIT diagnostic
algorithm, including the clinical scoring system, recall the role and limitations of
immunoassays for HIT, and apply the results of the serotonin release assay, also known
as SRA, to patient management.
Briefly, HIT is an important clinical pathologic syndrome.
HIT represents the most common drug induced immune mediated thrombocytopenia.
The incidence of HIT is common, and occurs in up to 3% of people who get exposed to therapeutic
doses of unfractionated heparin.
It can be difficult to confirm the diagnosis, and therefore management of HIT may be fairly
difficult and complex.
It is the most important drug induced thrombocytopenia due to the high thrombosis risk, with up to
50% of patients with HIT experiencing thrombosis.
We will start with a case example.
A 73 year old male was hospitalized for community acquired pneumonia.
In addition to receiving antibiotic therapy, he also received subcutaneous unfractionated
heparin for deep vein thrombosis prophylaxis.
This slide shows the clinical course of his thrombocytopenia.
On admission, he had a normal platelet count, and on about day five of heparin therapy the
platelet count started drifting down, and eventually reached a nadir on day seven of
heparin therapy.
At this point, the clinicians suspected HIT, discontinued heparin, and ordered the anti-heparin
platelet factor IV antibody immunoassay, which turned out to be strongly positive and he
was diagnosed as having isolated heparin induced thrombocytopenia.
Isolated HIT is not a benign condition.
Different studies demonstrate the incidence of thrombosis associated with HIT as ranging
between 18 to 52%, hence the importance of making an early and accurate diagnosis of
HIT so that optimal therapy can be provided in order to result in a good patient outcome.
HIT should be suspected in patients on heparin therapy who develop thrombocytopenia.
It can occur in those receiving both low molecular weight heparin and unfractionated heparin,
but the incidence is higher in those on unfractionated heparin.
In addition, HIT occurs in those on prophylactic and therapeutic doses of heparin.
The highest risk appears to be in patients receiving heparin boluses during cardiac bypass
procedures.
Perhaps the second most common situation is orthopedic surgery, such as joint arthoplasty.
Less commonly, HIT can occur in patients undergoing general surgery, and rarely in those receiving
thromboprophylaxis during hospitalization for medical indications and during pregnancy.
Recognition of HIT relies on selected clinical features.
This consists of assessing the degree and timing of onset of thrombocytopenia, presence
or absence of thrombosis, which can be venous or arterial or both.
In addition, it is important to exclude other potential causes of thrombocytopenia.
Assessing these clinical findings led to the development of a scoring system known as the
four Ts scoring system, which is very commonly used to assess the likelihood of HIT.
Based on the timing of onset of thrombocytopenia, the degree of thrombocytopenia, and presence
or absence of thrombosis, and exclusion of other potential explanations for the thrombocytopenia,
a four T score is generated that provides a likelihood of the patient having HIT.
As shown, a low score of three or less suggests a low probability of HIT, whereas a score
of six or higher suggests a high probability of HIT.
The final score is used in the calculation of a negative and positive predictive value
for HIT that assists in decision making process on the need for empiric change in anticoagulation
therapy and laboratory testing.
The first step in the diagnostic process for suspected HIT is to calculate the four T score.
A very well written systematic review and meta-analysis of the concluded that the negative
predictive value for a low probability four T score was 99.8%.
However, the positive predictive value for a non-low probability, in other words, a moderate
or high probability, was between 14 to 64%.
This data helps guide in the next steps.
A low clinical pretest probability has a negative predictive value of 99.8%.
Therefore, the current guideline recommendation is to avoid laboratory testing, and if a non-heparin
anticoagulant like a direct thrombin inhibitor was initiated, it can be discontinued and
the heparin may be continued or resumed.
Since a non-low probability four T score, which is also known as moderate or high probability,
does not have a very high positive predictive value, in this situation, heparin induced
thrombocytopenia testing should be ordered.
The most commonly available and widely ordered assay is an immunoassay.
Pending results of testing, however, heparin should be discontinued and a non-heparin anticoagulant
such as a direct thrombin inhibitor should be initiated.
In this setting of an intermediate or high probability of HIT, where an immunoassay result
has been ordered, and once the result is available and is negative, this provides a very high
negative predictive value for HIT.
At this stage, the non-heparin anticoagulant can be discontinued and heparin can be continued
or resumed if it was discontinued.
A positive immunoassay result suggests presence of antibodies directed against heparin platelet
factor IV complexes.
The next step is to ask the question, whether these antibodies are platelet activating,
or functional antibodies that are implicated in the pathogenesis of HIT.
Performance characteristics of immunoassays and functional assays are shown in this table.
Of the available functional assays, heparin induced platelet aggregation assay, also known
as the HIPAA, has been around for a long time.
However, the current gold standard functional assay is the serotonin release assay, also
known as the SRA, which is based on either radioactive or non-radioactive methodologies.
The relationship between a positive immunoassay test result and a positive SRA is shown in
this study.
A positive cutoff is at an optical density of 0.4.
As you can see, all patients with an optical density of 0.4 or less were negative for the
SRA assay.
However, as the optical density increased, there was a higher proportion of patients
with the positive SRA.
Close to 90% of patients with an optical density greater than or equal to two had a positive
SRA.
In the serotonin release assay implemented at Mayo Clinic, fresh platelets are obtained
from donors who are known to be reactive in this assay.
Platelets undergo standard washing process, which is fairly labor intensive.
The platelets are then incubated with the patient's serum, which contains the heparin
platelet factor IV antibodies, and two different concentrations of heparin, a low concentration
and a high concentration.
If the antibodies are platelet activating, platelet activation occurs, which releases
platelet contents.
Serotonin is one of the platelet contents, which is measured by liquid chromatography
tandem mass spectrometry, also known as LCMS.
Interpretation of the Mayo Clinic serotonin release assay is based on a cutoff of 20%
serotonin release.
Less than 20% serotonin release is a negative test result, however, greater than 20% serotonin
release, which is inhibited by high dose heparin, is a positive test result.
On occasion, there will be patients with antibodies that release serotonin at greater than 20%,
however, this release is not inhibited by heparin.
In other words, at a high heparin level, serotonin release is still greater than 20%.
This is categorized as an indeterminate test result, and not classical for heparin platelet
factor IV platelet activating antibodies.
Here is shown the correlation of the Mayo Clinic serotonin release assay with optical
density of the heparin platelet factor IV IgG ELISA assay.
As you can see, the large majority of patients with a high optical density, about 1.5, had
a positive serotonin release assay test result.
We will talk a little more detail about the one patient who had a negative test result.
On the other hand, the large majority of patients with a negative heparin platelet factor IV
ELISA had a negative serotonin assay test result.
In our experience, there was a variable degree of positivity of the SRA depending on the
optical density between 0.4 to 1.5 of the heparin platelet factor IV ELISA.
The one patient who had a positive ELISA with a very high optical density, but a negative
SRA assay test result, had a four T score that was moderate.
This patient developed post chemotherapy thrombocytopenia, and was receiving unfractionated heparin for
deep vein thrombosis prophylaxis.
The Mayo SRA and the send out test, which is radioactive methodology, were both negative.
This case represents an example of a non-platelet activating heparin platelet factor IV antibody.
The one indeterminate SRA test result in the group of patients with a negative heparin
platelet factor IV IgG test result was a patient with a new diagnosis of acute myeloid leukemia.
The patient was receiving unfractionated heparin DVT prophylaxis, and the four T score was
low.
Both the Mayo SRA and the mail out radioactive methodology based SRA were indeterminate.
The heparin was discontinued due to the induction chemotherapy related thrombocytopenia, and
there was no evidence of thrombosis.
What about the rare presentation of patients who have HIT, but are SRA negative?
There are reports of such patients who have a high clinical suspicion of HIT and a positive
HIT immunoassay test result, but a negative SRA.
These are confined to case reports and small case series.
Therefore, clinical evaluation is critical in the diagnosis and management of such patients.
In summary, heparin induced thrombocytopenia is a complex clinical pathologic syndrome.
Initial evaluation of patients with thrombocytopenia who are on heparin or have recently been exposed
to heparin should consist of a thorough clinical evaluation, assessment of a pretest clinical
probability known as the four T scoring system, and to follow the guidelines in follow up
testing and decisions on anticoagulation management.
The likelihood of HIT will influence the decision to interrupt heparin, to initiate non-heparin
anticoagulation, and whether or not to perform laboratory testing.
A positive HIT immunoassay test result should prompt confirmation of the pathogenicity of
the antibody in HIT.
Functional assay, which is a serotonin release assay, is currently gold standard for detection
of platelet activating antibodies.
Having said that, it should be kept in mind that there are rare cases of SRA negative
HIT, and cases of a negative ELISA based HIT it test result, but positive serotonin release
assay.
But, these are extremely rare.