A 62-year-old man is hospitalized for treatment of a pulmonary embolism and
is started on intravenous unfractionated heparin. On hospital day 6, he
develops new-onset pain and swelling of the left leg. He is afebrile and
hemodynamically stable. Laboratory studies show a decrease in platelet count
from 240,000/µL on admission to 100,000/µL. Hemoglobin and leukocyte counts are
within normal limits. Doppler ultrasound of the left lower extremity shows a
new deep vein thrombosis. There is no evidence of bleeding. He was hospitalized
2 months ago and received heparin at that time. Diagnosis?
Diagnosis is Heparin-Induced Thrombocytopenia with Thrombosis
(HITT).
1. Definition
Heparin-induced thrombocytopenia (HIT) is an immune-mediated,
highly prothrombotic adverse drug reaction caused by
platelet-activating IgG antibodies against platelet factor 4
(PF4)-heparin complexes.
It causes platelet activation, thrombocytopenia, marked thrombin
generation, and an increased risk of venous and arterial thrombosis.
When HIT is complicated by a new thrombosis, it is termed HIT with
thrombosis (HITT).
2. Clinical Forms
2.1 Nonimmune Heparin-Associated Thrombocytopenia
Historically termed type I HIT, although it is clinically
distinct from true immune HIT.
1. Nonimmune
2. Usually
develops within the first 1 to 4 days
3. Causes
a mild platelet count reduction
4. Is
transient
5. Is
not associated with the characteristic thrombotic syndrome of immune
HIT
2.2 Immune HIT
Historically termed type II HIT and represents true HIT.
1. Immune-mediated
2. Caused
primarily by IgG antibodies against PF4-heparin complexes
3. Typical
platelet fall begins 5 to 10 days after starting heparin
4. Associated
with a major risk of venous and arterial thrombosis
5. Requires
immediate discontinuation of heparin and initiation of alternative
anticoagulation
Rapid-onset HIT may develop within 24 hours of heparin
re-exposure when circulating HIT antibodies remain from a recent exposure. This
is most characteristic when heparin exposure occurred within the previous 30
days. Exposure 30 to 100 days previously can also be
relevant but is a weaker timing criterion.
In this patient, the platelet fall beginning on day 6
represents the classic timing of HIT despite his heparin exposure 2 months
earlier.
3. Pathophysiology
1. Platelet
factor 4 (PF4) released from platelet alpha granules binds heparin
2. PF4
and heparin form antigenic complexes
3. IgG
antibodies develop against PF4-heparin complexes
4. IgG-containing
immune complexes bind FcγRIIa receptors on platelets
5. Platelets
become strongly activated and release additional PF4
6. Activation
of platelets, monocytes, neutrophils, and coagulation pathways produces marked thrombin
generation
7. The
resulting hypercoagulable state causes venous and arterial
thrombosis
8. Platelet
activation, consumption, and clearance produce thrombocytopenia
HIT is therefore primarily a prothrombotic disorder, not a
bleeding disorder.
4. Clinical Features
4.1 Platelet Findings
1. A
platelet count fall >50% from the preceding peak is highly
characteristic
2. The
absolute platelet count may remain within the normal range despite a clinically
significant fall
3. The
median platelet nadir in HIT is approximately 55 × 10⁹/L
4. Severe
thrombocytopenia below 15 × 10⁹/L is unusual and should prompt
consideration of severe or autoimmune HIT, disseminated intravascular
coagulation, or another concomitant cause
5. Clinically
significant spontaneous bleeding is uncommon
The ≥20 × 10⁹/L threshold is particularly relevant to the 4Ts
scoring system rather than representing the typical platelet nadir in
HIT.
4.2 Thrombotic Manifestations
Venous thrombosis is generally more common than arterial thrombosis.
Common manifestations include:
1. Deep
vein thrombosis
2. Pulmonary
embolism
3. Limb-threatening
venous thrombosis
4. Arterial
limb ischemia
5. Ischemic
stroke
6. Myocardial
infarction
7. Other
organ thromboses
4.3 Other Manifestations
1. Skin
necrosis at heparin injection sites
2. Erythematous
skin lesions
3. Acute
systemic reaction after an intravenous heparin bolus, including fever,
chills, dyspnea, chest pain, tachycardia, or hypotension
5. Diagnosis
HIT is a clinical and laboratory diagnosis. Clinical
probability should be assessed first, followed by appropriate laboratory
testing.
5.1 4Ts Score
The 4Ts score is the recommended initial clinical assessment.
The four components are:
1. Thrombocytopenia
2. Timing
of platelet count fall
3. Thrombosis
or other HIT manifestations
4. Other
causes of thrombocytopenia
Interpretation:
1. 0
to 3: low probability
2. 4
to 5: intermediate probability
3. 6
to 8: high probability
A low-probability score generally excludes HIT sufficiently to avoid
HIT-specific laboratory testing unless the score is uncertain or important
clinical information is missing.
Intermediate or high clinical probability warrants laboratory evaluation and
appropriate immediate management.
5.2 4Ts Score in This Patient
1. Platelet
fall from 240,000/µL to 100,000/µL, a fall of approximately 58%,
with nadir ≥20,000/µL: 2 points
2. Clear
platelet fall on day 6: 2 points
3. New
confirmed DVT: 2 points
4. No
apparent alternative cause of thrombocytopenia: 2 points
Total 4Ts score = 8, indicating high clinical probability of HIT.
Therefore, heparin should be stopped and therapeutic non-heparin
anticoagulation started without waiting for laboratory confirmation.
5.3 PF4-Heparin Immunoassay
1. Detects
antibodies against PF4-heparin complexes
2. Has
high sensitivity but lower specificity
3. IgG-specific
immunoassays are generally more specific than polyspecific assays that
also detect IgA and IgM
4. Stronger
positive results, particularly higher ELISA optical density values, increase
the likelihood of clinically significant HIT
5. A
negative high-sensitivity immunoassay makes HIT very unlikely
If the 4Ts score is intermediate or high but a high-sensitivity immunoassay
is negative:
1. HIT
becomes very unlikely
2. Empiric
non-heparin anticoagulation being given specifically for suspected HIT can
generally be discontinued
3. Heparin
may be resumed if otherwise clinically indicated
4. If
the clinical picture remains strongly discordant with the negative result,
reassess the 4Ts score and consider repeat testing, an alternative
immunoassay, or functional testing, because rare false-negative
immunoassays can occur
5.4 Functional Assay
Functional assays determine whether the patient's antibodies actually activate
platelets in the presence of heparin.
1. Serotonin
release assay (SRA) is widely regarded as the gold-standard
functional assay
2. Heparin-induced
platelet activation assay (HIPA) is another validated functional assay
3. Functional
testing is generally performed after a positive immunoassay
when available
4. A
functional assay may not always be necessary when the clinical probability is
high and the immunoassay is very strongly positive
Diagnosis should integrate the 4Ts score, immunoassay result, assay
strength, and functional testing when indicated.
5.5 Screening for Occult Thrombosis
In patients with confirmed acute HIT without clinically apparent
thrombosis, screening for otherwise silent thrombosis is important.
1. Perform
bilateral lower-extremity compression ultrasonography
2. If
a central venous catheter is present, perform ultrasonography
of the catheterized upper extremity
3. Detection
of thrombosis changes the diagnosis from isolated HIT to HITT
and influences treatment duration
This patient already has a documented DVT, so additional screening for
occult thrombosis does not alter the diagnosis of HITT.
6. Management
6.1 Immediate Management
In a patient with high clinical probability of HIT:
1. Stop
all heparin immediately
2. Avoid
unfractionated heparin
3. Avoid
low-molecular-weight heparin
4. Stop
heparin-containing flushes and other heparin exposures
5. Start
an appropriate non-heparin anticoagulant
6. Do
not wait for laboratory confirmation before treating a
high-probability patient
Simply stopping heparin is insufficient because the thrombotic risk remains
high.
6.2 Non-Heparin Anticoagulation
Patients with acute HITT should receive therapeutic-intensity
anticoagulation unless a compelling contraindication exists.
Options include:
1. Argatroban
o Intravenous
direct thrombin inhibitor
o Short
half-life
o Useful
when rapid interruption of anticoagulation may be necessary
o Hepatically
cleared, so dose reduction may be required in hepatic impairment
2. Bivalirudin
o Intravenous
direct thrombin inhibitor
o Short
half-life
o Useful
when procedures may be required
o Commonly
used in cardiovascular procedural settings
o Dose
adjustment may be necessary with renal dysfunction
3. Fondaparinux
o Subcutaneous
factor Xa inhibitor
o Commonly
used as a non-heparin option in clinically stable patients
o Predominantly
renally cleared
o Fondaparinux-associated
HIT is extremely rare
o Avoid
or use an alternative in severe renal impairment
4. Direct
oral anticoagulants
o The
best contemporary evidence is for the oral factor Xa inhibitors rivaroxaban
and apixaban
o They
may be used as initial non-heparin anticoagulation in clinically stable
patients without severe limb or organ ischemia
o They
may be started before platelet recovery in appropriately
selected stable patients
o They
are generally avoided as initial therapy in hemodynamically unstable
patients or patients with severe limb or organ ischemia
o Their
use for acute HIT is off-label
o Selection
depends on renal function, hepatic function, bleeding risk, clinical stability,
thrombosis type, and drug interactions
Short-acting parenteral agents such as argatroban or bivalirudin
are particularly useful in critically ill patients, patients with high bleeding
risk, those with severe limb or organ ischemia, or those who may require urgent
procedures.
Fondaparinux, rivaroxaban, or apixaban may be suitable for clinically stable
patients when organ function and the clinical situation are appropriate.
6.3 Warfarin
Do not initiate warfarin during acute HIT before platelet recovery.
Early warfarin can cause rapid depletion of protein C and increase the risk
of:
1. Venous
limb gangrene
2. Skin
necrosis
If warfarin has already been started:
1. Stop
warfarin
2. Administer
vitamin K
3. Continue
appropriate non-heparin anticoagulation
If warfarin is ultimately selected, initiate it only after platelet
recovery, usually when the platelet count is ≥150,000/µL or has
returned to baseline.
Warfarin should be overlapped with an appropriate parenteral non-heparin
anticoagulant for at least 5 days and until the INR is
therapeutic.
Argatroban increases the INR independently of warfarin. Therefore,
transition from argatroban to warfarin must follow an argatroban-specific
transition protocol, and the combined INR should not be interpreted in
the same manner as an INR obtained with warfarin alone.
The parenteral overlap requirement does not apply when transitioning
appropriately to a DOAC.
6.4 Platelet Transfusion
Routine platelet transfusion is not recommended in acute
HIT because HIT is strongly prothrombotic.
Platelet transfusion may be considered when there is:
1. Active
significant bleeding
2. A
high risk of major bleeding
3. A
compelling procedural indication
6.5 Inferior Vena Cava Filter
Routine insertion of an inferior vena cava filter is not recommended
in acute HIT or HITT when therapeutic anticoagulation can be given.
7. Duration of Anticoagulation
7.1 HIT With Thrombosis
For HIT complicated by thrombosis:
1. 3
months of therapeutic anticoagulation is generally recommended when
HIT is the transient provoking factor
2. Duration
should be individualized when another indication for anticoagulation is present
In this patient, the duration must also account for the pulmonary
embolism for which he was originally hospitalized.
7.2 Isolated HIT Without Thrombosis
For confirmed HIT without thrombosis:
1. Continue
a non-heparin anticoagulant for at least 4 weeks or until the platelet
count is >150 × 10⁹/L, whichever is later
2. Routine
anticoagulation beyond this period is generally unnecessary unless there is persistent
HIT or another indication for anticoagulation
8. Complications
1. Deep
vein thrombosis
2. Pulmonary
embolism
3. Arterial
thrombosis
4. Acute
limb ischemia
5. Stroke
6. Myocardial
infarction
7. Venous
limb gangrene
8. Skin
necrosis
9. Limb
loss
10. Recurrent
or progressive thrombosis
11. Death in
severe untreated disease
9. Key Clinical Insight
Heparin exposure + platelet fall >50% beginning approximately 5
to 10 days later + new thrombosis strongly suggests immune HIT.
Calculate the 4Ts score first. In an intermediate- or
high-probability patient, stop heparin, obtain appropriate HIT testing, and
initiate non-heparin anticoagulation without waiting for confirmatory results
when treatment is indicated.
In this patient:
Day-6 platelet fall of 58% + new DVT + no clear alternative cause =
4Ts score 8 = high-probability HIT with thrombosis.
References
1. Arachchillage
DJ, Thachil J, Anderson JAM, et al. Diagnosis and management of
heparin-induced thrombocytopenia: Third edition. British Journal
of Haematology. 2024;204(2):459-475. doi:10.1111/bjh.19180.
2. Cuker
A, Arepally GM, Chong BH, et al. American Society of Hematology 2018
guidelines for management of venous thromboembolism: heparin-induced
thrombocytopenia. Blood Advances. 2018;2(22):3360-3392.
doi:10.1182/bloodadvances.2018024489.
3. May
J, Cuker A. Practical guide to the diagnosis and management of
heparin-induced thrombocytopenia. Hematology Am Soc Hematol Educ
Program. 2024;2024(1):388-395. doi:10.1182/hematology.2024000566.