Supportive and Preventive Medicine

HEP (HIT Expert Probability) score

Incorporates more clinical features than the 4T score

ii.

More complex (time-consuming) and evaluation studies limited (not in ICU patients)

Laboratory testing

Antigen assays

Depends on the detection of antibody binding by enzyme-linked immunosorbent assays or

particle-based immunoassays

ii.

An automated latex immunoturbidimetric assay has been made available. This assay does not

require sample batching as is common with the enzyme-linked immunosorbent assays; thus,

results can be obtained within 20 minutes.

iii.

Antibody present if sample from patient binds to the heparin-PF4–coated wells, leading to a

color-producing reaction. A higher antibody concentration leads to greater color production

and a higher optical density reading. Optical density readings of 0.4 or greater are considered

positive and indicative of HIT antibodies.

iv.

High sensitivity (greater than 90%) and low to moderate specificity

heparin 5–100 days earlier.

Functional assays

Examples: Heparin-induced platelet aggregation (HIPA) and C14 serotonin release assay

ii.

Detect platelet activation in the presence of heparin. Patient serum is mixed with washed

platelets from healthy volunteers and low and high concentrations of heparin. In the presence

of HIT antibodies, platelets are activated in low concentrations of heparin and detected either

visually (HIPA) or using radioactive serotonin (serotonin release assay).

iii.

Advantages: High sensitivity and specificity

iv.

Disadvantages: Technically challenging and not readily available; often send-out laboratory

tests with a turnaround time of several days

Treatment of HIT (Table 10)

An intermediate or high 4T score should prompt immediate discontinuation of all sources of heparin

and initiation of an alternative nonheparin anticoagulant until confirmatory testing is completed.

Parenteral direct thrombin inhibitors are the agents of choice for anticoagulation in acute HIT

because they have no cross-reactivity with heparin. Some studies support the use of the factor Xa

inhibitor fondaparinux for the treatment of HIT, though there are reports of fondaparinux-induced

HIT.

2007;41(5):749-754)

plus or minus 0.15 μg/kg/min in critically ill patients with multiple organ dysfunction.

in critically ill patients. In patients with severe liver impairment, consider 0.5 μg/kg/min.

ii.

iii.

Parenteral direct thrombin inhibitors are associated with a higher rate of major bleeding