Type of test Blood
Normal findings
Adult/elderly: 150,000-400,000/mm3 or 150-400 × 109/L (SI units)
Premature infant: 100,000-300,000/ mm3
Newborn: 150,000-300,000/ mm3
Infant: 200,000-475,000/ mm3
Child: 150,000-400,000/ mm3
Possible critical values
< 50,000 or > 1 million/mm3
Test explanation and related physiology
The platelet count is a count of the number of platelets (thrombocytes) per cubic milliliter of blood. It is performed on patients who develop petechiae (small hemorrhages in the skin), spontaneous bleeding, or increasingly heavy menses. It is also used to monitor the course of the disease or therapy for thrombocytopenia or bone marrow failure.
Platelet activity is essential to blood clotting. Counts of 150,000 to 400,000/mm3 are considered normal. Counts of less than 100,000/mm3 are generally considered to indicate thrombocytopenia; thrombocytosis (thrombocythemia) is generally said to exist when counts are greater than 400,000/ mm3. Common associative diseases with thrombocytosis are iron-deficiency anemia and malignancy (leukemia, lymphoma, or solid tumors, such as those involving the colon). Thrombocytosis may also occur with polycythemia vera, postsplenectomy syndromes, and a variety of acute/chronic infections or inflammatory processes.
Causes of thrombocytopenia (decreased number of platelets) include the following:
• Reduced production of platelets (secondary to bone mar row failure or infiltration of fibrosis, tumor, and so on)
• Sequestration of platelets (secondary to hypersplenism)
• Accelerated destruction of platelets (secondary to antibodies [see previous test], infections, drugs, prosthetic heart valves)
• Consumption of platelets (secondary to DIC)
• Platelet loss from hemorrhage
• Dilution with large volumes of blood transfusions that contain very few, if any, platelets
Interfering factors
• Living at high altitudes may cause increased platelet levels.
• Because platelets can clump together, automated counting is subject to at least a 10% to 15% error.
• Strenuous exercise may cause increased levels.
• Decreased levels may be seen before menstruation.
* Drugs that may cause increased levels include oral contraceptives.
* Drugs that may cause decreased levels include chemotherapeutic agents, chloramphenicol, colchicine, H2-blocking agents (cimetidine, ranitidine), hydralazine, indomethacin, isoniazid, levofloxacin, quinidine, streptomycin, sulfonamides, thiazide diuretics, and tolbutamide.
Procedure and patient care
• See inside front cover for Routine Blood Testing.
• Fasting: no
• Blood tube commonly used: lavender
• If the results indicate that the patient has a serious platelet deficiency, perform the following steps:
1. Observe the patient for signs and symptoms of bleeding.
2. Check for blood in the urine and all excretions.
3. Assess the patient for bruises, petechiae, bleeding from the gums, epistaxis, and low back pain.
4. Reassess all venipuncture sites for signs of hematoma formation.
Abnormal findings
Increased levels (thrombocytosis)
- Malignant disorder
- Polycythemia vera
- Postsplenectomy syndrome
- Rheumatoid arthritis
- Iron-deficiency anemia
Decreased levels (thrombocytopenia)
- Hypersplenism
- Hemorrhage
- Immune thrombocytopenia
- Leukemia and other myelofibrosis disorders
- Thrombotic thrombocytopenia
- Inherited thrombocytopenia disorders (e.g., Wiskott–Aldrich, Bernard–Soulier, or Zieve syndromes)
- DIC
- Systemic lupus erythematosus
- Pernicious anemia
- Some hemolytic anemias
- Cancer chemotherapy
- Acute or chronic infection
