Understanding Key Blood Parameters: Venous Blood, eGFR, MCV, WBC, and Platelet Count
By Dr Lauren Holland | Marylebone Diagnostic Centre, London
Blood testing is one of the most fundamental tools in modern medicine. It helps clinicians assess organ function, detect disease, and monitor overall health.
The following overview explains several important laboratory parameters often included in routine or specialised blood profiles.
Venous Blood
Venous blood is the blood collected from a vein, typically from the arm during a routine test. It carries deoxygenated blood from tissues back to the heart.
Why It’s Used in Testing
Venous blood provides a stable and representative sample for most biochemical and haematological investigations. It contains both cellular elements (red and white blood cells, platelets) and plasma, allowing laboratories to measure:
- Electrolytes (e.g., sodium, potassium)
- Renal and liver function markers
- Hormones and metabolites
- Blood cell counts and morphology
Venous samples are preferred over capillary blood when high precision is required, as they allow for better control of sample volume and anticoagulant balance.
eGFR – Estimated Glomerular Filtration Rate
eGFR (Estimated Glomerular Filtration Rate) is a calculated measure of how efficiently the kidneys filter waste products from the blood. It is derived from the serum creatinine level, age, sex, and ethnicity.
Clinical Significance
- eGFR is a key indicator of kidney function.
- A normal eGFR is generally above 90 mL/min/1.73 m².
- Values between 60–89 may indicate mild impairment, while values below 60 suggest a degree of chronic kidney disease (CKD).
Interpretation
It is important to interpret eGFR alongside other renal markers, such as urea and creatinine, and consider patient-specific factors including hydration, muscle mass, and medications that may affect renal filtration.
Mean Corpuscular Volume (MCV)
Mean Corpuscular Volume (MCV) represents the average size of red blood cells (erythrocytes). It is measured in femtolitres (fL) and forms part of the Full Blood Count (FBC).
Clinical Interpretation
- Low MCV (<80 fL): Suggests microcytic anaemia, often related to iron deficiency or thalassaemia.
- Normal MCV (80–100 fL): Indicates normocytic red cells, which may still occur in anaemia of chronic disease.
- High MCV (>100 fL): Reflects macrocytic anaemia, commonly caused by vitamin B12 or folate deficiency, liver disease, or excessive alcohol intake.
MCV is used in combination with other red cell indices (MCH and MCHC) to determine the cause of anaemia and guide further testing.
White Blood Cells (WBC)
White blood cells are essential components of the immune system. They defend the body against infections, inflammation, and foreign substances.
The WBC count measures the total number of white cells in a given volume of blood, typically expressed as x10⁹/L.
Clinical Significance
- High WBC count (leukocytosis): May indicate infection, inflammation, stress, or, in rare cases, haematological disorders.
- Low WBC count (leukopenia): Can occur due to viral infections, bone marrow suppression, or certain medications.
Differential Count
A full white cell differential divides the WBC into subtypes:
- Neutrophils: Fight bacterial infections.
- Lymphocytes: Regulate immune responses and viral defence.
- Monocytes: Remove debris and support immune signalling.
- Eosinophils: Involved in allergic reactions and parasitic infections.
- Basophils: Play a role in inflammatory and allergic responses.
Platelet Count
Platelets (thrombocytes) are small, disc-shaped cell fragments vital for blood clotting (haemostasis). The platelet count measures how many platelets are present in a microlitre of blood, usually ranging from 150–400 x10⁹/L.
Clinical Interpretation
- Low platelet count (thrombocytopenia): May result from viral infections, bone marrow disorders, autoimmune conditions, or certain medications.
- High platelet count (thrombocytosis): Can occur after inflammation, infection, or following blood loss. Persistent elevation may warrant investigation for myeloproliferative disorders.
Maintaining an adequate platelet count is crucial for wound healing and prevention of excessive bleeding.
Summary
| Parameter | Purpose | Typical Reference Range | Clinical Insight |
| Venous Blood | Primary sample type for laboratory testing | — | Provides stable and accurate biochemical and haematological data |
| eGFR | Kidney function assessment | >90 mL/min/1.73 m² | Indicates renal filtration efficiency |
| MCV | Red blood cell size | 80–100 fL | Helps diagnose anaemia type |
| WBC | Immune cell count | 4.0–11.0 x10⁹/L | Detects infection or immune dysfunction |
| Platelet Count | Clotting function | 150–400 x10⁹/L | Monitors bleeding or clotting risk |
Clinical Relevance at Marylebone Diagnostic Centre
At Marylebone Diagnostic Centre, each of these parameters is routinely measured within comprehensive health panels and blood profiles.
All tests are conducted to the Marylebone High Standard, ensuring precision, consistency, and clinically validated results.
Venous blood collection at 73 Baker Street is performed by trained professionals using modern laboratory systems designed for patient comfort and sample integrity.
