Vitreous Potassium PMI Calculator

Estimate post-mortem interval from temperature-independent potassium leakage in the vitreous humor.

mEq/L
Estimated Post-Mortem Interval
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PMI in Days
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Confidence
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How to Use This Vitreous Potassium PMI Calculator

  1. Obtain the potassium concentration from a forensic toxicology laboratory analysis of vitreous humor sampled from the deceased.
  2. Enter the value in mEq/L into the calculator. Typical post-mortem values range from about 5 mEq/L (recent death) to 20+ mEq/L (several days).
  3. Review the estimated PMI displayed in both hours and days, along with confidence notes about the estimate's reliability.

Understanding Vitreous Potassium and Time of Death

The vitreous humor is a clear, gel-like substance that fills the posterior chamber of the eye. After death, it becomes an invaluable forensic specimen because of its anatomical isolation — the eye's structure protects the vitreous from the bacterial contamination and putrefactive changes that rapidly affect blood, urine, and other body fluids. This isolation makes vitreous chemistry one of the most reliable post-mortem biochemical tools available to forensic pathologists.

The scientific basis of this method rests on a simple biological process: after death, cell membranes lose their ability to maintain ionic gradients. Potassium, which is normally concentrated inside cells at roughly 150 mEq/L compared to about 5 mEq/L in extracellular fluid, begins to leak outward. In the eye, retinal cells release their intracellular potassium into the vitreous humor at a remarkably linear and predictable rate.

Sturner's Linear Regression Model

This calculator uses the regression equation developed by Sturner and Gantner:

PMI (hours) = 7.14 × [K+] - 39.1

Where [K+] is the vitreous potassium concentration in milliequivalents per liter (mEq/L). The slope of 7.14 represents the average hours of post-mortem interval per unit increase in potassium, and the intercept of -39.1 accounts for the baseline potassium level in living vitreous humor (approximately 5.5 mEq/L).

Temperature Independence: A Key Advantage

Unlike body cooling methods such as Henssge's nomogram, which are heavily influenced by ambient temperature, air currents, clothing, and body composition, the vitreous potassium method is largely temperature-independent. The rate of potassium leakage from retinal cells is driven by cellular degradation and autolysis rather than thermodynamic heat exchange. This makes vitreous potassium analysis particularly valuable in cases where the body has been exposed to extreme temperatures, direct sunlight, or heating sources that would invalidate temperature-based PMI estimates.

Sample Collection and Laboratory Analysis

Vitreous humor is collected at autopsy (or at the scene in some jurisdictions) by inserting a sterile needle into the posterior chamber of the eye and aspirating the fluid. Typically 1-2 mL can be obtained from each eye. The sample is analyzed using flame photometry or ion-selective electrode methods in a forensic toxicology laboratory. Best practices call for sampling both eyes separately, as the average of two measurements improves accuracy.

Comparison to Body Cooling Methods

Forensic pathologists often use vitreous potassium analysis in conjunction with Henssge's body cooling method. When both methods yield similar PMI estimates, confidence in the result increases substantially. When the estimates diverge significantly, it may indicate that environmental factors (such as a fire, heating, or cooling system) affected body temperature after death but not the vitreous chemistry. This cross-validation approach represents best practice in forensic time-of-death estimation.

Valid Range and Limitations

The linear regression model is most accurate for potassium concentrations between approximately 5.5 and 20 mEq/L, corresponding to PMIs from near-zero to roughly 100 hours (about 4 days). Beyond this range, decomposition processes and potential vitreous liquefaction reduce reliability. Additionally, certain ante-mortem conditions — such as uremia, electrolyte imbalances, or prolonged agonal periods — may alter baseline potassium levels, introducing error into the PMI estimate.

Frequently Asked Questions

Vitreous humor is anatomically isolated within the eye, making it resistant to bacterial contamination and putrefactive changes that rapidly affect blood and other body fluids. The potassium concentration in vitreous humor rises linearly and predictably after death as intracellular potassium leaks through degrading cell membranes, providing a reliable biochemical clock for forensic investigation.
Vitreous humor is collected by inserting a needle into the posterior chamber of the eye and aspirating the fluid. The potassium concentration is then measured using flame photometry or ion-selective electrode analysis in a forensic toxicology laboratory. Typical values range from about 5 mEq/L shortly after death to over 20 mEq/L after several days.
Largely yes. Unlike body cooling methods, vitreous potassium concentration is minimally affected by ambient temperature. The rate of potassium leakage is driven primarily by cellular degradation rather than thermodynamic processes. This makes it especially useful when body temperature has already equilibrated with the environment, rendering cooling-based methods unreliable.
The formula is most reliable for potassium concentrations between approximately 5.5 and 20 mEq/L, corresponding to PMIs from near-zero to roughly 100 hours. Below 5.5 mEq/L, the result may be zero or negative (indicating very recent death). Above 20 mEq/L, decomposition and vitreous liquefaction reduce the linear model's reliability.
The two methods rely on entirely different biological processes — body cooling (thermodynamics) versus cellular degradation (biochemistry). When both yield similar PMI estimates, confidence is greatly increased. When they disagree, it may indicate unusual environmental conditions affected body temperature after death. Using both methods together is considered best practice in forensic pathology.