Introduction
Electrochemical blood glucose measurement is often described as a mature, well‑understood technology. Yet beneath the apparent simplicity of a disposable strip and a handheld meter lies a set of important assumptions that can materially affect analytical accuracy. One of the most significant — and frequently underappreciated — is hematocrit.
Hematocrit, the fraction of blood volume occupied by red blood cells, directly influences how much analyte is available for electrochemical conversion in small‑volume biosensors. If it is not accounted for correctly, the resulting measurement error can be substantial, even when the underlying chemistry is sound.
This article explores why hematocrit matters, how it affects electrochemical glucose measurements, and what modern strip designs do to compensate for it — with particular relevance to amperometric and coulometric biosensor architectures.
What Is the Hematocrit Effect?
Hematocrit is typically expressed as a percentage. A hematocrit of 40% means that 40% of the blood volume is occupied by red blood cells, with the remaining 60% being plasma.
From an electrochemical sensing perspective, this distinction matters because:
- Most dissolved analytes (such as glucose) are measured in the plasma
- Red blood cells act as excluded volume
- Analyte inside red blood cells is not immediately accessible to the electrode
As hematocrit increases, the effective plasma volume within a fixed sample volume decreases — even though the plasma concentration of the analyte remains unchanged.
Why Small‑Volume Glucose Strips Are Especially Sensitive
Modern self‑monitoring blood glucose (SMBG) strips operate with:
- Sample volumes on the order of hundreds of nanolitres
- Relatively large electrode areas
- High enzyme loading
Under these conditions, glucose is rapidly converted at the electrode surface. Although these systems are often described as amperometric, the reality is that many operate closer to coulometric mode, where:
✅ The total charge passed reflects the total amount of glucose in the sample, not just its instantaneous concentration.
This distinction is crucial when hematocrit changes.
A Simple Thought Experiment
Consider two samples with identical plasma glucose concentration:
- 5 mmol/L glucose in plasma
- Total sample volume: 300 nL
Case 1: 0% Hematocrit (Plasma Only)
- Plasma volume: 300 nL
- Total glucose present: proportional to full volume
- Total charge passed: ~300 µC (theoretical)
Case 2: 35% Hematocrit
- Plasma volume: ~195 nL
- Red blood cells exclude ~105 nL
- Plasma glucose concentration remains 5 mmol/L
- Total glucose available is reduced
- Total charge passed: ~195 µC
🔬 Key point: The instrument does not “see” concentration — it sees charge. Less accessible glucose means less charge, even though plasma concentration has not changed.
The Consequence: Apparent Under‑Reading
If hematocrit is ignored:
- A 35% hematocrit sample would appear to have ~3.25 mmol/L glucose
- A 50% hematocrit sample could appear as ~2.5 mmol/L
- At very high hematocrit, errors scale roughly linearly
📉 This creates a systematic under‑reading at higher hematocrit values, which is unacceptable in clinical or point‑of‑care settings.
How Modern Glucose Strips Compensate
Blood glucose strip manufacturers are well aware of this phenomenon. Compensation typically involves measuring hematocrit directly within the strip and correcting the glucose signal accordingly.
Common approaches include:
- Impedance measurements
- Conductivity or resistance measurements
- Correlation of electrical properties with red blood cell volume fraction
Once hematocrit is estimated, the meter can:
- Adjust the measured charge
- Back‑calculate the true plasma glucose concentration
- Present a corrected result to the user
✅ When a glucose meter reports “5 mmol/L,” it is often the result of two coupled measurements: glucose conversion and hematocrit estimation.
Implications for Biosensor Design
For engineers and researchers developing electrochemical biosensors, the hematocrit effect highlights several broader lessons:
⚙️ Volume Matters
At nanolitre scales, excluded volume effects dominate. Assumptions that hold in millilitre‑scale electrochemistry often break down.
🔬 Mode of Measurement Is Critical
Coulometric systems are inherently sensitive to total analyte amount. Without compensation, they are vulnerable to matrix effects like hematocrit.
💡 Integrated Sensing Is a Strength
Strips that combine:
- Electrochemical analyte detection
- Electrical characterization of the sample
are far more robust than single‑parameter designs.
Zimmer & Peacock’s experience with capillary‑fill screen‑printed electrodes and biosensor platforms reflects this integrated approach, particularly in applications where sample variability cannot be controlled.
For readers interested in electrode formats relevant to this discussion, see:
- Screen‑printed electrodes:
https://shop.zimmerpeacock.com/en-gb/collections/bare-electrodes
Key Practical Takeaways
📌 Hematocrit does not change glucose concentration — it changes accessible volume
📌 Coulometric signals scale with total analyte, not concentration
📌 Ignoring hematocrit leads to systematic under‑reading
📌 Modern glucose strips actively measure and compensate for hematocrit
📌 Biosensor designers must treat hematocrit as a first‑order variable
Closing Thoughts
The hematocrit effect is a powerful reminder that biosensors do not measure chemistry in isolation — they measure chemistry within a physical system. Volume exclusion, transport limitations, and matrix effects can all shape the final signal.
Understanding these effects is essential not only for glucose monitoring, but for any electrochemical sensor operating in complex biological fluids at small volumes.
If you are exploring biosensor development, electrochemical measurement strategies, or strip‑based diagnostics and would like to discuss design considerations in more depth, the Zimmer & Peacock team is always happy to engage.