What is the Full Blood Count (FBC)?
The Full Blood Count (FBC), also known as Complete Blood Count (CBC), is the most commonly ordered blood test worldwide. It provides a comprehensive overview of the cells circulating in your blood — red blood cells, white blood cells, and platelets. A single FBC test gives your doctor a wealth of information about your overall health.
What Does the FBC Measure?
The FBC measures several components of your blood:
- White Blood Cells (WBC): Fight infection and are part of your immune system
- Red Blood Cells (RBC): Carry oxygen from lungs to the rest of the body
- Hemoglobin (Hb): The oxygen-carrying protein inside red blood cells
- Hematocrit (HCT/PCV): The percentage of blood volume made up of red blood cells
- Platelets: Help blood clot and stop bleeding
- MCV (Mean Corpuscular Volume): Average size of your red blood cells
- MCH (Mean Corpuscular Hemoglobin): Average amount of hemoglobin per red blood cell
- MCHC (Mean Corpuscular Hemoglobin Concentration): Average concentration of hemoglobin in red blood cells
Why is This Test Done?
- As part of a routine health check-up
- To investigate fatigue, weakness, or feeling unwell
- To diagnose anemia (low hemoglobin)
- To detect infections (bacterial, viral, parasitic)
- To monitor dengue fever — platelet count and hematocrit are critical
- To screen for blood disorders such as thalassemia trait
- To monitor the effects of medications or chemotherapy
- Before surgery to check your baseline blood values
How to Prepare for the FBC Test
Simple Preparation
- No fasting required. You can eat and drink normally before the test.
- The test can be done at any time of the day.
- A small blood sample is drawn from a vein in your arm into a purple/EDTA tube.
- The procedure takes less than 5 minutes.
- Inform your doctor if you are taking blood thinners or any other medications.
Understanding Your FBC Results
| Component | Normal Range (Adults) | What It Tells You |
|---|---|---|
| White Blood Cells (WBC) | 4,000 - 11,000 /µL | High = infection, inflammation. Low = viral infection, bone marrow issue. |
| Red Blood Cells (RBC) | M: 4.5-5.5 million/µL F: 3.8-4.8 million/µL |
Low = anemia. High = dehydration, polycythemia. |
| Hemoglobin (Hb) | M: 13.5-17.5 g/dL F: 12.0-15.5 g/dL |
The key value for diagnosing anemia. Low Hb = anemia. |
| Hematocrit (HCT/PCV) | M: 38-50% F: 36-44% |
Rising HCT in dengue = plasma leakage (danger sign). |
| Platelets | 150,000 - 400,000 /µL | Low = bleeding risk (dengue, bone marrow disorders). High = inflammation, infection. |
| MCV | 80 - 100 fL | Low = iron deficiency, thalassemia. High = vitamin B12/folate deficiency. |
| MCH | 27 - 33 pg | Low = iron deficiency anemia. High = macrocytic anemia. |
| MCHC | 32 - 36 g/dL | Low = iron deficiency. Helps classify type of anemia. |
FBC in Dengue Fever
The FBC is the single most important test in monitoring dengue fever. Your doctor will order daily FBCs from Day 3 of fever onwards.
| Parameter | What Happens in Dengue | Why It Matters |
|---|---|---|
| Platelet Count | Drops progressively (can fall below 10,000) | Low platelets increase bleeding risk. Below 50,000 requires close monitoring. |
| Hematocrit (HCT) | Rises as plasma leaks out of blood vessels | Rising HCT (>20% increase from baseline) indicates plasma leakage — a danger sign. |
| White Blood Cells | Usually low (leukopenia) | Low WBC is characteristic of dengue and helps confirm the diagnosis. |
FBC in Common Conditions in Sri Lanka
- Dengue monitoring: Daily FBC from Day 3 of fever is the standard of care. Platelet drop and rising hematocrit are the key parameters to watch.
- Iron deficiency anemia in women: Very common in Sri Lanka, especially in women of reproductive age. FBC shows low hemoglobin, low MCV (small red cells), and low MCH. Often related to heavy menstrual periods and dietary iron deficiency.
- Thalassemia trait: Sri Lanka has a significant prevalence of beta-thalassemia trait. The FBC shows low MCV and MCH with normal or slightly low hemoglobin. It is often mistaken for iron deficiency anemia. Proper identification is important for genetic counselling before marriage.
- Infections: Bacterial infections typically show high WBC, while viral infections (dengue, chikungunya) often show low WBC.
- Chronic kidney disease: Anemia is common in CKD patients, particularly those in the North Central Province affected by CKDu.
What Should You Do Next?
- Normal FBC: No specific action needed. Recheck as part of your annual health screening.
- Low hemoglobin (anemia): Your doctor will investigate the cause — iron deficiency, vitamin B12 deficiency, thalassemia trait, or chronic disease. Do not self-treat with iron tablets.
- Low platelets: If you have dengue, follow your doctor's monitoring plan closely. If found incidentally, further investigation is needed.
- Abnormal WBC: Your doctor will correlate with your symptoms. A differential count may be ordered to see which type of white cells are affected.
Important: The FBC is one of the most informative blood tests, but results must be interpreted in context with your symptoms, medical history, and other investigations. Never self-diagnose based on FBC results alone. Always consult your doctor for proper interpretation.