Photo-Optical Principle Explains Increased Plasma Opacity During Clot Formation
The correct answer is C. Photo-optical principle, which explains how plasma becomes more opaque/turbid as a clot forms, leading to increased light absorption.
How Photo-Optical Clot Detection Works
Photo-optical clot detection technology relies on measuring changes in light transmission or absorbance that occur during the process of clot formation 1. This principle is the foundation for modern automated coagulation analyzers that monitor clotting reactions.
There are two main types of optical clot detection systems:
Transmittance-based systems:
- Monitor decreases in light transmittance during clot formation
- Transmittance decreases after initiation of clotting
- Examples include MDA-II and CS series analyzers 2
Absorbance-based systems:
- Monitor increases in absorbance during clot formation
- 0% absorbance defines the pre-coagulation phase
- Absorbance increases after clotting begins
- Examples include ACL series analyzers 2
The Physical Basis for Increased Opacity
During the clotting process, fibrinogen (a soluble protein in blood and plasma) is converted into an insoluble mesh of fibrin polymers. This transformation causes the clear yellow plasma to become turbid and non-transparent 2. The formation of this fibrin mesh physically changes the optical properties of the plasma by:
- Increasing light scattering
- Reducing light transmission
- Increasing light absorption
As the Journal of Thrombosis and Haemostasis guidelines explain, the clot waveform can be divided into distinct phases 1:
- Pre-coagulation phase
- Coagulation phase (where opacity increases)
- Post-coagulation phase
Advanced Analysis Using Optical Data
Modern analyzers don't just measure the final opacity but analyze the entire curve pattern:
- First derivatives of the transmittance/absorbance curves reflect coagulation velocity
- Second derivatives reflect coagulation acceleration 2, 1
These parameters provide valuable information about clotting function and can be used to detect subtle abnormalities in the coagulation process.
Clinical Applications
The photo-optical principle has several important clinical applications:
- Routine coagulation testing (PT, APTT)
- Assessment of global coagulation function
- Detection of coagulation abnormalities
- Monitoring anticoagulant therapy 3
Important Caveats
- Plasma turbidity: Pre-existing turbidity in plasma (e.g., from lipemia) can affect optical clot detection results 4, 5
- Lyophilized plasma: Reconstituted lyophilized plasmas are often more turbid than normal plasmas due to agglomeration of lipids, which can affect test results 5
- Plasma substitutes: Certain colloid solutions can reduce clot quality, which may be detected by optical methods 6
Understanding the photo-optical principle is essential for interpreting coagulation test results accurately and recognizing potential interferences that might affect test reliability.