[Health Lecture] Do you know the reason for low platelet count?

Platelets play an important role in physiological and pathological processes such as hemostasis, wound healing, inflammatory response, thrombosis and organ transplant rejection. Platelets are small pieces of cytoplasm that fall off from the cytoplasm of mature megakaryocytes in the bone marrow and are released into the blood. They have the function of maintaining the integrity of the vascular endothelium, and have the functions of adhesion, aggregation, release, coagulation and blood clot contraction, playing an important role in thrombosis and hemostasis.

Platelets are mostly regular round, with diameters ranging from 2 to 3 microns. Due to individual differences, large platelets or deformed platelets can also be seen in some people. Platelets can move and deform, so they appear in multiple forms when observed. Clinically, thrombocytopenia is a common cause of bleeding.

The platelet count in normal adult blood circulation is (100-300)×10^9/L. Usually, the platelet count in China is < 100x10^9/L, and the platelet count internationally is < 150×10^9/L, which is defined as thrombocytopenia. A platelet count of < 30×10^9/L is clinically reported as a critical platelet value.

The common causes of thrombocytopenia are physiological, blood diseases and the influence of some medications, but in daily work, the laboratory often encounters patients whose platelet count results are far lower than the actual level, and the platelet histogram shows abnormalities, and the system cannot accurately calculate the platelet volume. In this case, we first consider whether there is a false decrease in platelets in the specimen due to aggregation, or because the platelet volume is too large and is counted into other blood cells, causing a false decrease. If at this time, the laboratory fails to accurately identify the false decrease in platelets and issues a report, it can easily lead to clinical incorrect diagnosis and treatment decisions.

So, what are the common causes of false low platelets?

1. False thrombocytopenia caused by blood collection

Smooth blood collection, unskilled techniques, and long collection time due to thin blood vessels can all cause tissue factor to enter the blood sample and produce tiny clots that are invisible to the naked eye. If the collector is not familiar with the process, the blood is not fully mixed after inversion or the sample is placed directly without mixing, it will affect the anticoagulant effect of EDTA, thereby causing clots and (or) platelet aggregation. If the ratio of blood volume to anticoagulant is not appropriate, platelets will be diluted or the anticoagulant effect will be unsatisfactory. Therefore, in daily work, it is necessary to constantly supervise and be familiar with the business process. Not only must the ratio of blood volume to anticoagulant be ensured, but also attention must be paid to timely inversion after blood collection and sufficient mixing to reduce pseudo-thrombocytopenia caused by improper blood collection.

2. EDTA-dependent pseudothrombocytopenia

EDTA anticoagulation is the standard anticoagulant for routine blood collection tubes. EDTA-dependent pseudothrombocytopenia is an in vitro phenomenon caused by antiplatelet autoantibodies in the presence of EDTA, which cause platelet aggregation. It directly targets hidden antigenic determinants, which are usually hidden in the platelet membrane protein (Gp) Ⅱb/Ⅲa. Since (Gp) Ⅱb/Ⅲa needs to maintain its heterodimer structure in the presence of calcium ions, EDTA can separate (Gp) Ⅱb/Ⅲa through its chelation with calcium ions, resulting in the exposure of (Gp) Ⅱb/Ⅲa clusters, binding to autoantibodies in plasma, activating phospholipase A2 and phospholipase C in cells, hydrolyzing platelet membrane phospholipids and releasing active substances such as arachidonic acid, ADP, 5-HT, prothrombin, endogenous calcium ions, etc. These active substances can activate platelet and fibrinogen receptors, causing platelets and fibrinogen to aggregate into clusters.

3. False decrease caused by large (giant) platelets interfering with the test

Normally, the size or volume of platelets detected by a blood cell analyzer is between 2-30 fl. When the platelet volume is too large, exceeding 25 fl, the size of some platelets reaches the threshold of red blood cell detection, and the instrument will mistakenly identify them as red blood cells, causing the aggregated platelets to not be counted in the platelet channel, interfering with the red blood cell counting results. The instrument mistakenly identifies the platelets as small red blood cells or red blood cell fragments, resulting in a false decrease in platelets.

4. False platelet decrease caused by cold agglutination samples

In most cases, cold agglutination only causes a false decrease in red blood cell count without affecting platelet count; however, a small number of patients have high titer of cold agglutinins in their bodies. High titer cold agglutinins can agglutinate red blood cells and nucleated cells and platelets in the blood at low temperatures, resulting in a significant decrease in platelets and red blood cells in instrument test results, and smears showing platelet aggregation distribution.

5. Drug-induced thrombocytopenia

Long-term use of drugs such as angiotensin, digoxin, and magnesium sulfate can cause drug-induced thrombocytopenia. In hyperlipidemia, platelet aggregation increases, and some patients may develop pseudothrombocytopenia. Patients with diabetes and hypertension have fragile vascular endothelium, which makes platelets easily aggregated and difficult to dissolve, resulting in low platelet values ​​detected by instruments.

After understanding the common causes of false low platelets, if we encounter low platelets in our daily work, how can we promptly eliminate the above factors and provide clinicians with accurate and reliable test results?

1. If the blood routine test results show that the platelet count is low, and the impedance method instrument indicates platelet aggregation or abnormal platelet histogram, first observe the specimen, gently invert and mix the blood collection tube to observe whether there are clots or tiny agglutinations visible to the naked eye. If agglutination is present, contact the clinic in time and resample the patient.

2. If it is cold agglutination, the specimen should be tested on the machine immediately after warm bath. If necessary, the specimen should be collected at the machine and tested immediately.

3. If there are no clots or microagglutinations visible to the naked eye, the specimen should be smeared, stained and examined under a microscope to confirm whether platelets can be seen aggregated or clustered under the microscope. If platelet aggregation is visible under the microscope, the clinic should be notified in time to resample the patient and collect additional EDTA tubes and sodium citrate anticoagulant tubes (blue tubes) for simultaneous inspection to rule out pseudo-thrombocytopenia caused by blood collection or EDTA.

4. If platelets are seen in clusters or clumps under the microscope, platelet detection can also be performed through the reticulocyte channel. The reticulocyte channel uses staining + warm bath + oscillation technology to disaggregate aggregated platelets and re-form the platelets into a dispersed state, thereby correcting false platelet reduction and obtaining a platelet count result close to the patient's actual level.

5. If no aggregation is found under microscopic examination, but large (giant) platelets are seen, we can take the following measures:

(1) Manual platelet counting.

(2) Platelet estimation method summarized by Professor Zhang Shimin:

Platelet count (×10^9/L) ≈ average number of platelets per oil immersion field × 15 (×10^9/L)

At present, there are many reasons for false platelet decrease, which are often encountered in daily testing work. When the test personnel find that the instrument test results are inconsistent with the patient's clinical manifestations or symptoms and signs, they should first consider the situation of false platelet decrease, combine the platelet histogram and alarm prompts, timely smear staining microscopy, combined with manual counting and other methods to find a reasonable explanation, correct the false platelet decrease, and provide reliable test data for clinical treatment. And communicate closely with the clinic, ask about the medical history and medication, and re-draw blood for reexamination when necessary to avoid misleading the clinic due to the false platelet decrease caused by the above reasons and taking inappropriate treatment.

About the Author

Cai Xiaofeng

Chief Technician of the Laboratory Department of Qinghai Provincial Hospital of Traditional Chinese Medicine

Member of the Communist Party of China. Has been engaged in laboratory work for 15 years, majoring in clinical laboratory.