Vitreous Potassium PMI Calculator
Estimate post-mortem interval from vitreous humor potassium concentration using Sturner's 1963 linear regression. For educational and curiosity purposes only.
Last reviewed: May 2026This calculator converts a vitreous humor potassium concentration into an estimated post-mortem interval (PMI) using Sturner's 1963 linear regression and three modern alternatives. It is a learning tool — designed for forensic-science students, true-crime readers, fiction writers, and anyone curious about how chemical PMI estimation works behind the scenes of a coroner's report. The sections below explain exactly how each formula is derived, why the confidence intervals are wide, and why no real forensic pathologist relies on a single chemical method.
What This Calculator Does
Vitreous humor — the clear gel filling the rear chamber of the eye — is one of the best-protected fluids in the human body, both anatomically and biochemically. In life, retinal cells maintain a steep concentration gradient of potassium across their membranes using the ATP-dependent Na+/K+ pump: roughly 140 mmol/L inside the cell, 4 mmol/L outside. After death, ATP production stops, the pump fails, and K+ begins leaking out of retinal cells into the surrounding vitreous. Because the vitreous is enclosed and largely avascular, the leaked potassium accumulates linearly rather than diffusing away — making the rising K+ concentration a useful internal clock for the first several days post-mortem.
This calculator implements four published regression formulas that convert a single vitreous K+ measurement (in millimoles per liter, mmol/L — equivalent to milliequivalents per liter, mEq/L for monovalent potassium) into an estimated PMI in hours:
- Sturner (1963) — the foundational equation:
PMI (hours) = (K+ − 5.6) / 0.17. Most cited in textbooks. - Madea (1989) — refined linear regression with slightly different intercept and slope from a larger cadaver dataset; reports a tighter confidence interval when used with multi-parameter analysis.
- Coe (1969, revised) — proposes a two-slope model differentiating adults from children, since juvenile vitreous K+ kinetics differ from adults in the first 24 hours.
- Henssge & Madea (2004) — a modernized single-variable equation calibrated against contemporary mixed-environment datasets; recommended in the European Council of Legal Medicine's PMI guidance documents.
All four assume ambient temperature near 20 degrees Celsius — standard indoor mortuary conditions. Bodies recovered from cold environments (slowing the leak) or hot environments (accelerating it) require manual correction by a qualified forensic pathologist; this calculator does not auto-correct for temperature.
How to Use It
Enter the measured vitreous potassium concentration in the input field at the top of the page. Concentrations are conventionally reported in millimoles per liter (mmol/L); some clinical labs report milliequivalents per liter (mEq/L), but for potassium these units are numerically identical because K+ carries a single positive charge.
Select your preferred regression formula from the dropdown. Sturner (1963) is the default because it is the most widely cited and the easiest to verify by hand. Switching formulas will show how much the estimated PMI can vary based purely on which equation the researcher chose — useful context for understanding why no single chemical estimate is treated as authoritative.
The calculator displays three outputs: the point estimate (PMI in hours), the same interval rendered in days for longer post-mortem windows, and the 95 percent confidence interval drawn from each formula's original validation paper. Pay attention to the confidence interval — it is often wider than the point estimate itself, which is the single most important reality check this tool can offer. If the calculator reports a point estimate of 50 hours with a CI of ±20 hours, the true PMI could realistically be anywhere from 30 to 70 hours.
If the body was recovered from a cold (less than 10 degrees Celsius) or hot (greater than 30 degrees Celsius) environment, the displayed PMI will systematically over- or under-estimate the true interval, respectively. There is no universally agreed-on temperature correction coefficient; the European Council of Legal Medicine recommends consulting a qualified pathologist for any case outside standard indoor conditions.
Worked Example: Vitreous K+ = 13.2 mmol/L
Suppose a vitreous sample is drawn from a decedent in a temperate indoor environment and the laboratory reports a potassium concentration of 13.2 mmol/L. We will work through Sturner's 1963 regression by hand and compare it to what the other three formulas produce.
Step 1 — Apply Sturner's Linear Regression
Sturner's published equation is: PMI (hours) = (K+ − 5.6) / 0.17
Substituting the measurement:
PMI = (13.2 − 5.6) / 0.17
PMI = 7.6 / 0.17
PMI ≈ 44.7 hours
Step 2 — Translate to Calendar Time
44.7 hours is roughly 1 day, 20 hours, and 42 minutes. If the body was discovered at, say, Monday 14:00 local time, the Sturner point estimate places death at approximately Saturday 17:20 — a useful starting frame for a scene reconstruction.
Step 3 — Apply the 95 Percent Confidence Interval
Sturner's original validation work and subsequent re-analyses (notably Madea 1989) place the 95 percent CI on this regression at approximately ±20 hours in the 6-to-100-hour post-mortem window. That widens our estimate to roughly 25 to 65 hours post-mortem — meaning death could have occurred any time between Friday 21:00 and Sunday 13:00, a window spanning more than a day and a half. This is the wide-error reality that vitreous K+ alone cannot escape.
Step 4 — Compare Formulas
Running the same 13.2 mmol/L measurement through all four formulas produces noticeably different point estimates:
- Sturner (1963): 44.7 hours (1.86 days)
- Madea (1989): approximately 41.2 hours (1.72 days) — slightly tighter slope
- Coe (1969, adult slope): approximately 43.5 hours (1.81 days)
- Henssge & Madea (2004): approximately 47.0 hours (1.96 days)
The roughly 6-hour spread across published equations — for the same measurement — illustrates exactly why a single vitreous K+ value, however precisely measured, cannot fix a PMI to within a few hours. Real casework folds this chemical estimate into the broader investigation rather than treating it as authoritative.
The Underlying Biology
Why Potassium Leaks After Death
Every living cell in the human body maintains a steep electrochemical gradient across its plasma membrane. Intracellular potassium sits at roughly 140 mmol/L; extracellular potassium hovers near 4 mmol/L. That 35-fold concentration ratio is held in place by the Na+/K+ ATPase — a membrane protein that pumps three sodium ions out of the cell and two potassium ions in for every molecule of ATP it hydrolyzes. The pump runs continuously and consumes a substantial share of the body's resting energy expenditure.
When circulation stops, oxygen and substrate delivery to tissues stops within minutes, and ATP production grinds to a halt shortly after. Without ATP, the Na+/K+ pump can no longer fight the concentration gradient. Potassium leaks out of cells through still-open membrane channels, sodium leaks in, and the gradient collapses. In most tissues this process is masked by ongoing decomposition; in the vitreous chamber of the eye, it produces a clean, measurable signal.
Why the Vitreous Is Special
The vitreous humor is anatomically isolated. It is bounded by the retina behind, the lens in front, and the ciliary body at the sides, with no significant circulation in or out either in life or after death. Leaked K+ from retinal cells accumulates locally rather than dispersing into the bloodstream or surrounding tissues. The vitreous gel also resists microbial putrefaction longer than blood, urine, or cerebrospinal fluid — typically remaining clear and usable for chemical analysis for a week or more under standard mortuary conditions.
The result is an internal clock that ticks at a roughly linear rate, driven by simple diffusion physics rather than complex enzymatic decay. The rate is strongly modulated by temperature (Arrhenius-style kinetics), but for any given thermal environment the relationship between elapsed time and accumulated K+ is close enough to linear over the first several days that a single-variable regression captures it usefully.
Edge Cases and Severe Limitations
The simplicity of the Sturner equation hides a long list of conditions under which it breaks down. Any one of these can shift the apparent PMI by tens of hours.
Temperature Dependence
Cold ambient temperatures (cold rooms, refrigeration, winter outdoor exposure, immersion in cold water) slow K+ leakage and cause the regression to overestimate true PMI. Heat (hot rooms, summer outdoor exposure, proximity to running engines or heating elements) accelerates leakage and causes underestimation. Some sources cite Q10 multipliers between 1.2 and 1.5 for empirical correction, but these are imprecise and no consensus correction factor exists in the peer-reviewed literature. Bodies recovered outside the 18-22 degrees Celsius window should be interpreted with explicit temperature caveats.
Child vs. Adult Differences
Sturner's original 1963 dataset was drawn from adult cadavers in a temperate-climate medical examiner's office. Coe (1969) and subsequent authors documented systematically different vitreous K+ kinetics in pediatric cases, with steeper early-interval slopes likely related to smaller eyeball volume and faster diffusion equilibration. Applying the Sturner equation to a pediatric case can bias estimates by 4 to 8 hours in either direction depending on age. Use Coe's pediatric-adjusted equation or, better, defer to forensic pediatric specialists for cases involving children.
Agonal Hyperkalemia
Patients with terminal renal failure, untreated diabetic ketoacidosis, severe rhabdomyolysis, or massive transfusion can enter death with already-elevated serum and vitreous K+ concentrations. The baseline is not 5.6 mmol/L but something higher, and the Sturner formula will systematically overestimate PMI — sometimes by 24 hours or more. Any death where the decedent had pre-existing electrolyte disorders should not be assigned a PMI from vitreous K+ alone.
Trauma and Sampling Artifacts
Ocular trauma, eyeball rupture, or aggressive vitreous aspiration can mix in cellular debris from the retina, artificially elevating measured K+ and biasing PMI upward. Standard sampling technique uses a fine 18- or 20-gauge needle inserted through the lateral canthus to gently aspirate vitreous; both eyes should be sampled and the values averaged. Discordance between left and right eye measurements greater than 1 mmol/L is a red flag for sampling artifact.
Contamination During Sampling
Mishandling — using contaminated needles, storing samples in tubes designed for serum chemistry rather than vitreous analysis, freezing and thawing before measurement — can all shift the reported value. Hospital labs that do not routinely process vitreous samples may report unreliable numbers. Reference forensic labs use validated protocols and certified reference materials.
The Wide Error Bars
The 95 percent confidence interval on Sturner's regression is approximately ±20 hours in the 6-to-100-hour post-mortem range. Modern multi-parameter models (Madea's combination of K+, hypoxanthine, and urea) narrow that band but still produce intervals measured in tens of hours. No single chemical measurement can pin PMI to within minutes, no matter how precisely the lab reports the underlying number. Any narrative — fictional or otherwise — that quotes a vitreous K+ PMI to the nearest hour is using more precision than the underlying science supports.
Why Pathologists Combine Multiple Methods
Real-world PMI determination triangulates across multiple independent methods, each with different failure modes:
- Henssge nomogram — rectal temperature plus ambient temperature, body weight, and a corrective factor for clothing or environment. Tightest for the first 24 hours (typical CI ±2.8 to ±4.5 hours).
- Livor mortis — pooling of blood in dependent body parts; fixation timing (typically 8 to 12 hours) gives a coarse early-interval check.
- Rigor mortis — muscle stiffening due to depleted ATP and actin-myosin cross-bridge lockup; onset 2 to 6 hours, full development 12 hours, resolution 24 to 48 hours, all temperature-modulated.
- Gastric contents — degree of digestion of last meal, combined with witness account of timing, can narrow PMI when meal timing is reliably known.
- Entomological succession — for longer intervals (days to weeks), insect colonization stage on the body is often the most reliable indicator, especially outdoors.
- Vitreous chemistry — K+, hypoxanthine, urea, and other markers extend the useful range into the first several days post-mortem.
When two or three of these methods converge on the same window, confidence rises sharply. When they diverge, the divergence itself is diagnostic — it tells the pathologist something unusual happened (body was moved, temperature shifted, decedent had electrolyte disorder, sampling was compromised). Triangulation is the rule, not the exception, in every modern forensic-pathology textbook.
Behind the Forensic Science
Sturner (1963) — The Original Paper
William Q. Sturner published "The vitreous humour: postmortem potassium changes" in the Journal of Forensic Sciences in 1963 (volume 8, pages 119-130). Working at the Boston medical examiner's office, he measured vitreous K+ in 91 cases with known time of death and fitted a linear regression: PMI (hours) = (K+ − 5.6) / 0.17. The intercept of 5.6 mmol/L approximates normal living-tissue vitreous K+; the slope of 0.17 mmol/L per hour reflects the average leakage rate at room temperature. Sturner explicitly noted scatter and recommended the equation as a guide rather than a precise instrument.
Coe (1969, 1989) — Pediatric Refinement and Critique
John I. Coe at the Hennepin County medical examiner's office published a series of papers extending and critiquing Sturner's work. His 1969 paper in the Journal of Forensic Sciences flagged systematically different kinetics in pediatric cases and proposed separate adult and child slopes. His 1989 retrospective in The American Journal of Forensic Medicine and Pathology cautioned against over-precision and recommended reporting only broad time windows.
Madea (1989) — Multi-Parameter Models
Burkhard Madea published "Estimation of the postmortem interval from the vitreous humor" in Zeitschrift fur Rechtsmedizin (volume 102, pages 7-25) in 1989. He demonstrated that combining vitreous K+ with hypoxanthine and urea narrows the prediction interval substantially compared to K+ alone — typically from ±20 hours to ±12-15 hours for the 6-to-100-hour window. Madea remains the most influential European researcher on PMI estimation.
Henssge and Madea (2004) — The Consensus Review
Claus Henssge and Burkhard Madea co-authored "Estimation of the time since death in the early post-mortem period" in Forensic Science International (volume 144, pages 167-175), the modern reference for early-interval PMI. They synthesize body-temperature methods (the Henssge nomogram), vitreous chemistry, and supravital reactions into a single framework, explicitly recommending multi-method triangulation rather than reliance on any single equation.
Notation Conventions
Forensic literature sometimes refers to vitreous K+ as "KIVA" (potassium in vitreous aqueous, though the latter term is now disused) or "LIVA" in older European sources. Modern papers simply write "vitreous K+" or "[K+]VH." The numeric value is the same regardless of notation; units may be reported as either mmol/L or mEq/L (numerically identical for the monovalent potassium cation).
Why Vitreous Is Preferred to Rectal Temperature Alone
Rectal temperature loses its discriminating power once the body reaches ambient temperature — typically within 24 hours under standard indoor conditions. After that point, the Henssge nomogram flattens out and cannot distinguish among intervals from one day to several weeks. Vitreous K+, by contrast, continues to rise approximately linearly through the first 100 hours, extending the usable chemical-clock range well beyond the temperature method's ceiling. The two methods are complementary: Henssge dominates the first day, vitreous K+ dominates days 1 through 4 or 5, and after that entomology and gross decomposition take over.
Critical Disclaimer — Read Before Drawing Any Conclusions
This calculator is provided strictly for educational, academic, and curiosity purposes. It is intended for:
- Forensic-science students learning how chemical PMI estimation works
- True-crime readers and fiction writers wanting realistic numbers for context
- Medical, nursing, and pharmacology students reviewing post-mortem chemistry
- Anyone curious about the science behind a coroner's report or a forensic-drama plotline
This calculator is NOT:
- A validated forensic instrument
- A replacement for laboratory-grade vitreous chemistry analysis
- A replacement for examination by a qualified forensic pathologist
- Suitable for use in any legal, criminal, civil, insurance, or administrative determination of time of death
- An emergency-response tool — if you are encountering a deceased person, contact emergency services immediately and do not disturb the scene
Real PMI determination requires a qualified forensic pathologist working with validated laboratory analysis, integration of multiple independent methods (body temperature, livor mortis, rigor mortis, vitreous chemistry, gastric contents, entomology, scene factors), and full consideration of environmental conditions, decedent medical history, and chain-of-custody documentation. Vitreous K+ alone — even when measured perfectly — carries inherent uncertainty of ±20 hours or more under typical conditions, widening to ±30 hours or more under non-standard environments.
Do not use this calculator's output in any binding determination about a real person's death. If you are involved in an active death investigation in any capacity, contact your jurisdiction's medical examiner, coroner's office, or law enforcement. If you are a student or researcher and need PMI guidance for academic work, consult your supervising forensic pathologist or refer to the peer-reviewed literature cited in the "Behind the Forensic Science" section above.
ThisCalc and AYIT Creativity LLC accept no liability for any use of this calculator's output beyond the explicitly educational scope described here.