Thrombocytopenia usually has no symptoms and is picked up on a routine full blood count (or complete blood count). Some individuals with thrombocytopenia may experience external bleeding such as nosebleeds, and/or bleeding gums. Some women may have heavier or longer periods or breakthrough bleeding. Bruising, particularly purpura in the forearms and petechiae in the feet, legs, and mucous membranes, may be caused by spontaneous bleeding under the skin.

Eliciting a full medical history is vital to ensure the low platelet count is not secondary to another disorder. It is also important to ensure that the other blood cell types, such as red blood cells and white blood cells, are not also suppressed.

Painless, round and pinpoint (1 to 3 mm in diameter) petechiae usually appear and fade, and sometimes group to form ecchymoses. Larger than petechiae, ecchymoses are purple, blue or yellow-green areas of skin that vary in size and shape. They can occur anywhere on the body.

A person with this disease may also complain of malaise, fatigue and general weakness (with or without accompanying blood loss). Acquired thrombocytopenia may be associated with a history of drug use. Inspection typically reveals evidence of bleeding (petechiae or ecchymoses), along with slow, continuous bleeding from any injuries or wounds. Adults may have large, blood-filled bullae in the mouth. If the person's platelet count is between 30,000 and 50,000/mm, bruising with minor trauma may be expected; if it is between 15,000 and 30,000/mm, spontaneous bruising will be seen (mostly on the arms and legs).

Symptoms usually present from the period of birth to early childhood as: nose bleeds, bruising, and/or gum bleeding. Problems later in life may arise from anything that can cause internal bleeding such as: stomach ulcers, surgery, trauma, or menstruation. Abnormality of the abdomen, Epistaxis, Menorrhagia, Purpura, Thrombocytopenia, and prolonged bleeding time have also been listed as symptoms of various Giant Platelet Disorders.

Giant platelet disorders can be further categorized:

- caused by auto-immune disorders, for example Immune thrombocytopenic purpura (ITP), and characterized by low platelet count, but high MPV (Mean-Platelet Volume).

- Caused by glycoprotein abnormalities: Bernard-Soulier syndrome, Velocardiofacial syndrome

- Caused by calpain defect: Montreal platelet syndrome

- Caused by alpha granules defect: Gray platelet syndrome

- Characterized by abnormal neutrophil inclusions: May-Hegglin anomaly, Sebastian syndrome

- With systemic manifestations: Hereditary macrothrombocytopenia with hearing loss, Epstein syndrome, Fechtner syndrome

- With no specific abnormalities: Mediterranean macrothrombocytopenia

- Harris platelet syndrome

High platelet levels do not necessarily signal any clinical problems, and are picked up on a routine full blood count. However, it is important that a full medical history be elicited to ensure that the increased platelet count is not due to a secondary process. Often, it occurs in tandem with an inflammatory disease, as the principal stimulants of platelet production (e.g. thrombopoietin) are elevated in these clinical states as part of the acute phase reaction.

High platelet counts can occur in patients with polycythemia vera (high red blood cell counts), and is an additional risk factor for complications.

A very small segment of patients report symptoms of erythromelalgia, a burning sensation and redness of the extremities that resolves with cooling and/or aspirin use.

Scientific literature sometimes excludes thrombocytosis from the scope of thrombophilia by definition, but practically, by the definition of thrombophilia as an increased predisposition to thrombosis, thrombocytosis (especially primary thrombocytosis) is a potential cause of thrombophilia. Conversely, secondary thrombocytosis very rarely causes thrombotic complications.

Thrombocytopenia is a condition characterized by abnormally low levels of thrombocytes, also known as platelets, in the blood.

A normal human platelet count ranges from 150,000 to 450,000 platelets per microliter of blood. These limits are determined by the 2.5th lower and upper percentile, so values outside this range do not necessarily indicate disease. One common definition of thrombocytopenia requiring emergency treatment is a platelet count below 50,000 per microliter.

Immune thrombocytopenia (ITP) is a type of thrombocytopenic purpura defined as isolated low platelet count (thrombocytopenia) with normal bone marrow and the absence of other causes of thrombocytopenia. It causes a characteristic purpuric rash and an increased tendency to bleed. Two distinct clinical syndromes manifest as an acute condition in children and a chronic condition in adults. The acute form often follows an infection and has a spontaneous resolution within two months. Chronic immune thrombocytopenia persists longer than six months with a specific cause being unknown.

ITP is an autoimmune disease with antibodies detectable against several platelet surface antigens.

ITP is diagnosed by a low platelet count in a complete blood count (a common blood test). However, since the diagnosis depends on the exclusion of other causes of a low platelet count, additional investigations (such as a bone marrow biopsy) may be necessary in some cases.

In mild cases, only careful observation may be required but very low counts or significant bleeding may prompt treatment with corticosteroids, intravenous immunoglobulin, anti-D immunoglobulin, or immunosuppressive drugs. "Refractory ITP" (not responsive to conventional treatment) may require splenectomy, the surgical removal of the spleen. Platelet transfusions may be used in severe bleeding together with a very low count. Sometimes the body may compensate by making abnormally large platelets.

Signs include the spontaneous formation of bruises (purpura) and petechiae (tiny bruises), especially on the extremities, bleeding from the nostrils and/or gums, and menorrhagia (excessive menstrual bleeding), any of which may occur if the platelet count is below 20,000 per μl. A very low count (<10,000 per μl) may result in the spontaneous formation of hematomas (blood masses) in the mouth or on other mucous membranes. Bleeding time from minor lacerations or abrasions is usually prolonged.

Serious and possibly fatal complications due to extremely low counts (<5,000 per μl) include subarachnoid or intracerebral hemorrhage (bleeding inside the skull or brain), lower gastrointestinal bleeding or other internal bleeding. An ITP patient with an extremely low count is vulnerable to internal bleeding caused by blunt abdominal trauma, as might be experienced in a motor vehicle crash. These complications are not likely when the platelet count is above 20,000 per μl.

Increased platelet counts can be due to a number of disease processes:

- Essential (primary)

- Essential thrombocytosis (a form of myeloproliferative disease)

- Other myeloproliferative disorders such as chronic myelogenous leukemia, polycythemia vera, myelofibrosis

- Reactive (secondary)

- Inflammation

- Surgery (which leads to an inflammatory state)

- Hyposplenism (decreased breakdown due to decreased function of the spleen)

- Splenectomy

- Asplenia (absence of normal spleen function)

- Iron deficiency anemia or hemorrhage

Over-medication with drugs that treat thrombocytopenia, such as eltrombopag or romiplostim, may also result in thrombocytosis.

Other causes include the following

- Kawasaki disease

- Soft tissue sarcoma

- Osteosarcoma

- Dermatitis (rarely)

- Inflammatory bowel disease

- Rheumatoid arthritis

- Nephritis

- Nephrotic syndrome

- Bacterial diseases, including pneumonia, sepsis, meningitis, urinary tract infections, and septic arthritis.

The vast majority of causes of thrombocytosis are acquired disorders, but in a few cases, they may be congenital, such as thrombocytosis due to congenital asplenia.

HPS was identified among healthy blood donors in the north-eastern part of the Indian subcontinent, characterized by absent bleeding symptoms, mild to severe thrombocytopenia (platelets rarely <50 X 109/L)with giant platelets (Mean platelet volume 10fL) and normal platelet aggregation studies with absent MYH9 mutation.

In the blood donors with HPS authors found a statistically higher MPV, RDW and a lower platelet count and platelet biomass.

At present the diagnosis of HPS is made by ascertaining the ethnicity of the patient, as well as assessing for conditions causing acquired thrombocytopenias, and after also excluding the known inherited giant platelet disorders(IGPD) and other congenital thrombocytopenias. Unfortunately some patients with IGPD are treated inappropriately with corticosteroids, immunoglobulin infusions and even splenectomy.

It is extremely important to recognize Harris platelet syndrome, as one third the population of certain parts of Indian subcontinent is affected.

Bernard–Soulier syndrome often presents as a bleeding disorder with symptoms of:

Harris platelet syndrome (HPS) is the most common inherited giant platelet disorder.

The differential diagnosis for Bernard–Soulier syndrome includes both Glanzmann thrombasthenia and pediatric Von Willebrand disease. BSS platelets do not aggregate to ristocetin, and this defect is not corrected by the addition of normal plasma, distinguishing it from von Willebrand disease.

X-linked thrombocytopenia is typically diagnosed in infancy. The disease presents as a bleeding disorder with easy bruising, mucosal bleeding, such as nosebleeds, and mild to severe anemia. Anemia is a condition in which there is an insufficient number of red blood cells to carry adequate levels of oxygen to the body’s tissues. X-linked thrombocytopenia is considered to be the milder phenotype of the "WAS"-related disorders. As age increases, the severity of symptoms tends to decrease. However, individuals with X-linked thrombocytopenia have an increased risk for life-threatening brain hemorrhages and spontaneous bleeding.

Diagnosis is done by the help of symptoms and only blood count abnormality is thrombocytopenia.

Post-transfusion purpura (PTP) is an adverse reaction to a blood transfusion or platelet transfusion that occurs when the body produces alloantibodies to the introduced platelets' antigens. These alloantibodies destroy the patient's platelets leading to thrombocytopenia, a rapid decline in platelet count. PTP usually presents 5–12 days after transfusion, and is a potentially fatal condition.

PTP is rare, but usually occurs in women who have had multiple pregnancies or in people who have undergone previous transfusions. The precise mechanism leading to PTP is unknown, but it most commonly occurs in individuals whose platelets lack the HPA-1a antigen (old name: PL). The patient develops antibodies to the HPA-1a antigen leading to platelet destruction. In some cases, HPA-5b has also been implicated. It is unclear why alloantibodies attack the patient's own, as well as the introduced platelets. Probable explanation for this is that the recipient's platelet acquire the phenotype of donor's platelet by binding of the soluble antigens from the donor onto the recipient's platelet. It is usually self-limiting, but IVIG therapy is the primary treatment. Plasmapheresis is also an option for treatment.

Heparin may be used for both prevention and the treatment of thrombosis. It exists in two main forms: an "unfractionated" form that can be injected under the skin or through an intravenous infusion, and a "low molecular weight" form that is generally given subcutaneously (administered under the skin). Commonly used low molecular weight heparins are enoxaparin, dalteparin, nadroparin and tinzaparin.

In HIT, the platelet count in the blood falls below the normal range, a condition called thrombocytopenia. However, it is generally not low enough to lead to an increased risk of bleeding. Most people with HIT will therefore not experience any symptoms. Typically the platelet count will fall 5–14 days after heparin is first given; if someone has received heparin in the previous three months, the fall in platelet count may occur sooner, sometimes within a day.

The most common symptom of HIT is enlargement or extension of a previously diagnosed blood clot, or the development of a new blood clot elsewhere in the body. This may take the form of clots either in arteries or veins, causing arterial or venous thrombosis, respectively. Examples of arterial thrombosis are stroke, myocardial infarction ("heart attack"), and acute leg ischemia. Venous thrombosis may occur in the leg or arm in the form of deep vein thrombosis (DVT) and in the lung in the form of a pulmonary embolism (PE); the latter usually originate in the leg but migrate to the lung.

In those receiving heparin through an intravenous infusion, a complex of symptoms ("systemic reaction") may occur when the infusion is started. These include fever, chills, high blood pressure, a fast heart rate, shortness of breath, and chest pain. This happens in about a quarter of people with HIT. Others may develop a skin rash consisting of red spots.

Frequently, the thrombocytopenia is mild and the affected neonates remain largely asymptomatic. In these cases, therapeutic interventions are not indicated. In case of severe thrombocytopenia, the neonates may exhibit hemorrhagic complication at or a few hours after delivery. The most serious complication is intracranial hemorrhage, leading to death in approximately 10% or neurologic sequelae in 20% of cases.

Evans syndrome is an autoimmune disease in which an individual's antibodies attack their own red blood cells and platelets. Both of these events may occur simultaneously or one may follow on from the other.

Its overall pathology resembles a combination of autoimmune hemolytic anemia and immune thrombocytopenic purpura. Autoimmune hemolytic anemia is a condition in which the red blood cells that normally carry oxygen and carbon dioxide are destroyed by an autoimmune process. Immune thrombocytopenic purpura is a condition in which platelets are destroyed by an autoimmune process. Platelets are a component of blood that contribute to the formation of blood clots in the body to prevent bleeding.

The syndrome was first described in 1951 by R. S. Evans and colleagues.

Thrombocytopenic purpura are purpura associated with a reduction in circulating blood platelets which can result from a variety of causes, such as kaposi sarcoma.

The primary manifestations are thrombocytopenia and megakaryocytopenia, or low numbers of platelets and megakaryocytes. There is an absence of megakaryocytes in the bone marrow with no associated physical abnormalities.

The clinical hallmark is haemorrhagic bullae on the mucosa of the oronasopharynx. Haemorrhage from ruptured bullae, epistaxis or gastrointestinal bleeding is severe and may cause shock and death.

Onyalai is an acute disease that results in the development of hemorrhagic lesions on oral, nasal, and subconjunctival mucous membranes and skin, including on the soles of the feet. The patient initially is not in distress, which may result in a delay of diagnosis. As the disease progresses, hematuria, melena and menorrhagia may develop. Bleeding usually persists for approximately eight days, and may recur. Approximately 80 percent of cases will develop chronic thrombocytopenia. Periodic episodes of acute hemorrhage are possible and may be severe, leading to shock and death.

Heparin-induced thrombocytopenia (HIT) is the development of thrombocytopenia (a low platelet count), due to the administration of various forms of heparin, an anticoagulant. HIT predisposes to thrombosis (the abnormal formation of blood clots inside a blood vessel) because platelets release microparticles that activate thrombin, thereby leading to thrombosis. When thrombosis is identified the condition is called heparin-induced thrombocytopenia and thrombosis (HITT). HIT is caused by the formation of abnormal antibodies that activate platelets. If someone receiving heparin develops new or worsening thrombosis, or if the platelet count falls, HIT can be confirmed with specific blood tests.

The treatment of HIT requires stopping heparin treatment, and both protection from thrombosis and choice of an agent that will not reduce the platelet count any further. Several alternatives are available for this purpose and mainly used are danaparoid, fondaparinux, argatroban and bivalirudin.

While heparin was discovered in the 1930s, HIT was not reported until the 1960s.

The cause for this disorder appears to be a mutation in the gene for the TPO receptor, "c-mpl", despite high levels of serum TPO. In addition, there may be abnormalities with the central nervous system including the cerebrum and cerebellum which could cause symptoms.

X-linked thrombocytopenia, also referred to as XLT or thrombocytopenia 1, is an inherited clotting disorder that primarily affects males. It is a "WAS"-related disorder, meaning it is caused by a mutation in the Wiskott-Aldrich Syndrome ("WAS") gene, which is located on the short arm of the X chromosome. "WAS"-related disorders include Wiskott-Aldrich syndrome, XLT, and X-linked congenital neutropenia (XLN). Of the "WAS"-related disorders, X-linked thrombocytopenia is considered to be the milder phenotype. Between 1 and 10 per million males worldwide are affected with this disorder. Females may be affected with this disorder but this is very rare since females have two X chromosomes and are therefore typically carriers of the mutation.