Impact-R, a test for platelet function, has potential use as adjunctive diagnostic test to identify protein C or S deficiency.

A blood clot.

What is protein C or S deficiency?

Protein C and S are proteins found in the plasma that participate in the coagulation cascade as regulators of thrombin formation by inhibition of Factor VIIIa and Factor Va. Protein C is activated by thrombin-thrombomodulin complex then combines with free Protein S, its cofactor, to inhibit Factor VIIIa and Factor Va.

Inherited protein C or S deficiency is rare and occurs independently of each other. Protein C or S abnormalities include decreased levels of either protein, inability to bind with each other, protein C/S complex that that cannot degrade factors VIIIa and Va, increased clearance of Protein S.

Deficiencies of either protein C or S results in non-inhibition of factors VIIIa and Va and continuous, unchecked formation of thrombin. This leads to hypercoagulability of patient’s blood, and may result in deep vein thrombosis, venous thromboembolism, purpura fulminans or disseminated intravascular coagulation.

What are the tests for protein C or S deficiency?

Laboratory tests include Functional Protein C, Functional Protein S, Protein C Antigen and Protein S Antigen. These are specific for the Protein deificiency.

Adjunctive tests include Bleeding time, Partial Thromboplastin time, Prothrombin time and Thrombin time.

What is the use of Impact-R in diagnosing protein C or S deficiency?

Impact-R can be of use in the diagnosis of protein C or S by determining the effect of adding free protein S and activated protein C in the platelet adhesion and aggregation of whole blood of patients with the suspected deficiency. This can be an alternative in cases where the Protein C or S assays are not available.

Impact-R, a cone and plate analyzer for platelet adhesion and platelet aggregation may be utilised as a screening test for bleeding disorders such as Hemophilia A.

What is Hemophilia A?

Hemophilia A is a blood disorder that causes prolonged bleeding due to a deficiency of blood clotting Factor VIII.

Hemarthrosis of the elbow joint.

Symptoms are spontaneous bleeding, bruising, blood in the urine, gastrointestinal bleeding, epistaxis, prolonged bleeding from cuts and following surgeries such as circumcision and dental extraction, hemarthroses or bleeding into joints such as the knee. Symptoms may not be apparent in infants but will soon show when the baby starts to crawl or walk. Severity of the symptoms may vary. Major bleeding could be fatal such as those that happen if the patient is involved in a motor vehicular accident or sports-related injuries.

Hemophilia A is a hereditary disorder that is X-linked recessive. The disease manifests only in males because of their single X chromosome, whereas, females have two X-chromosomes. Very rarely, females manifest the disease if both X chromosomes are affected. This happens when the father is a haemophiliac and the mother is a carrier. Affected males mated to non-carriers will have male offspring that do not have the disease and female offspring that are carriers. Carrier females mated to un-affected males will have 50% chance of having male children with the disease and 50% chance of having female children who will be carriers.

The Use of Impact-R

Impact-R could be of use as a bedside screening test in children whose mothers have male relatives that have Hemophilia. It is a simple and rapid test that can determine defects in coagulation affecting platelet function. Studies have to be done, however, to determine the positive and negative predictive value of Impact-R as a screening test for Hemophilia A.

The Impact-R machine provides high-shear stress conditions needed by ADAMTS-13 to cleave ULVWF multimers seen in Thrombotic Thombocytopenic Purpura.

Skin lesions in TTP.

Thrombotic thrombocytopenic purpura is a rare disorder of the coagulation cascade. The etiology of TTP includes that of inhibition of function or the deficiency of ADAMTS-13, a metalloprotease enzyme that cleaves von Willebrand factor multimers in high shear stress conditions. If it is inhibited or deficient, the vWF forms ultra large multimers that predispose to thrombi formation in the microvasculature by spontaneously attaching to the platelet membrane GpIba.

A study by Jing Fei Dong, et.al.demonstrates this phenomenon. In the study,

Beads-on-a-string formation of platelets and ULVWF multimers.

cells with GpIba receptors in their membranes and platelets formed long beads-on-a-string formations on the endothelial culture cells when exposed to ultra large von Willebrand factor (ULVWF) multimers under high shear stress conditions.

When blood from a patient with TTP is tested using the Impact-R, surface coverage and aggregate size are expected to be large because of the presence of ULVWF in the patient’s plasma. When the same blood is exposed to plasma from a normal patient (which contains ADAMTS-13), and then tested by the Impact-R, theoretically, there will be a lessening of the surface coverage and aggregate size. This is the subject of a study by Shenkman in which he proposed that Impact-R could be used as a rapid screening test for TTP.

Impact-R determines the degree of platelet adhesion and platelet aggregation under near physiologic conditions. Platelet adhesion is expressed as %SC or the percentage of the surface covered by platelet aggregates. Platelet aggregation is expressed as AS µm² or the average size of the aggregates.

Ristocetin.

Sometimes, a platelet agonist is needed to assay platelet function in cases where there is defective adhesion and aggregation. One of these agonists is Ristocetin. Ristocetin is an antibiotic previously used to treat staphylococcal infections. Use was discontinued because of its lethal side-effects, thrombocytopenia and platelet agglutination. Because of these same side-effects, ristocetin is now used to diagnose certain hematologic conditions such as von Willebrand disease and Bernard-Soulier syndrome.

Ristocetin causes von Willebrand factor to bind the platelet receptor GpIb so that when ristocetin is added to whole blood, it agglutinates. The mechanism for such activity is yet unexplained but there are several hypotheses including one in which the binding of ristocetin to the platelet changes its surface charge. Ristocetin will show hypoagglutination in von Willebrand disease because of a deficiency in von Willebrand factor and in Bernard-Soulier syndrome because of a deficiency in GpIb receptor proteins in the platelet cell membrane.

Tests that use ristocetin include: Ristocetin Cofactor Activity and Ristocetin Induced Platelet Aggregation.

Thrombin is a powerful platelet agonist.

Impact R is a useful tool to assess platelet function in different hematologic diseases and to monitor anti-platelet therapy.  In the study by Shenkman, et.al., Impact-R was used to test agonist-induced platelet aggregation.

Some of the known platelet agonists are: ADP, epinephrine, arachidonic acid, gamma-thrombin, and collagen. In the aforementioned study, they used ADP, ristocetin, epinephrine, collagen and arachidonic acid. These agonist act through various mechanisms that affect different stages of the coagulation process.

The Impact-R agonist response test detected platelet aggregation defects in patients with storage pool disease, von Willebrand disease and epinephrine response deficiency.  It may be useful in determining response to various platelet agonists.

This is a case of a 79-year old man with severe aortic stenosis and recurrent GI bleeding. This case study by Schmid, et.al. proposes that Impact-R to determine platelet function may be more time- and cost-effective compared to electrophoretic analysis of vWF multimers.

Severe aortic stenosis with bleeding diathesis is also known as Heyde syndrome. The pathophysiology: loss of high molecular weight vWF multimers in high shear stress conditions leads to a platelet type von Willebrand disease (acquired vWD, vWD type 2B) which can explain the bleeding tendencies of a patient with severe aortic stenosis.

Along with other laboratory equipment, the Impact-R has been used in a study by Vaduqanathan, et.al. to evaluate platelet reactivity in patients with metabolic syndrome.

Metabolic syndrome is a combination of several medical conditions that increase the risk for developing cardiovascular disease and diabetes mellitus. It is also known as Syndrome X, insulin resistance syndrome, and Reaven’s syndrome.

The diagnostic criteria for metabolic syndrome (as defined by International Diabetes Foundation) includes central obesity with any two of the following:

–          Raised triglycerides

–          Reduced HDL cholesterol

–          Raised blood pressure

–          Raised fasting plasma glucose

Metabolic syndrome is associated with a prothrombotic state and platelet reactivity can therefore be considered a result. The study by Vaduqanathan, et.al. evaluated the platelet reactivity of patients with metabolic syndrome and their response to aspirin. They concluded that metabolic syndrome increases baseline platelet reactivity and lowers platelet response to aspirin.

Platelet apheresis concentrates are an invaluable blood product in the treatment of many disorders involving platelet deficiency. Congenital diseases such as Glanzmann’s thrombasthenia, Platelet type von Willebrand disease, and May-Heggelin anomaly, as well as acquired diseases such as Dengue Hemorrhagic Fever rely on platelet transfusions.

A previous article focused on the testing of platelet concentrates as they are stored in the laboratory facility. A study by Horvath, et.al., however, tested the patient’s blood for platelet function after a platelet transfusion has been done. In many cases, the effect of a platelet transfusion is quantified as an increase in the platelet count and not the platelet function.

The use of Impact-R as a diagnostic and monitoring tool for platelet function is feasible as was concluded by Horvath, et.al.

Chediak-Higashi syndrome can also be found in killer whales.

Chediak-Higashi syndrome (CHS) is a rare autosomal recessive mutation in the LYST gene. This results in microtubule defects in the cell that causes decreased phagocytosis. The decreased phagocytic activity of the cells in the body leads to recurrent pyogenic infections, skin pigmentation disorders and peripheral neuropathy. Manifestations include neutropenia, oculocutaneous albinism, periodontal disease, and bleeding disorders.

The bleeding disorders are heterogeneous. A study of 8 CHS patients by Al-Sheyyab, et.al. pointed out several mechanisms for the bleeding diathesis in CHS. In one patient, there is abnormal aggregation due to storage pool deficiency (SPD) of ADP and serotonin. In another patient, the aggregation defect resembles that of von Willebrand disease type IIb. Other defects involved the dense bodies and the alpha-granules as well as the platelet membrane.

An avenue of research in the pathophysiology of CHS is therefore opened up by these findings. What are the different platelet defects associated with CHS? What are the different mechanisms leading to an abnormality in platelet aggregation? What is the effect of ristocetin in platelet aggregation and how is this significant in CHS patients. The use of Impact-R will greatly aid the researcher in the investigation of the abovementioned issues.

What is a screening test? It is a device used to detect disease or a precursor of a disease in persons who do not have signs and symptoms. The decision to screen should be based on a careful history and assessment of risk factors.

Children with a family history of bleeding disorders may well benefit from a

Alexei Nikolaevich, heir to the Russian throne, had Hemophilia B.

screening test. The child’s future health and quality of life will depend on the diagnosis of a serious medical condition ahead of the time when signs and symptoms manifest. Children with a family history of congenital platelet function defects, clotting factor deficiencies, and defects in primary or secondary aggregation are among those that will put a screening test to good use.

A study by Revel-Vilk, et.al. evaluated the use of the Impact-R as a one such screening test. They concluded that the Impact-R is highly effective in excluding a bleeding disorder in patients with normal result. However, in case of abnormal Impact-R results, further testing is necessary to diagnose the condition.