Blog

The lethal triad now has a fourth corner. How strong is the evidence for the “lethal diamond”?

20 October 2026 · Echelon Medical Solutions

For decades, trauma teams have used the "lethal triad" to describe how a bleeding patient can spiral: hypothermia, acidosis and coagulopathy, each making the other two worse. In recent years a fourth element, low calcium, has been proposed, turning the triad into a "lethal diamond". It is an attractive idea, and it has already started to shape practice. This article looks at what the studies actually show, including where the evidence for the fourth corner is weaker than the name suggests.

The lethal triad in brief

The triad describes a vicious cycle. Acidosis and hypothermia together promote coagulopathy, a failure of the blood to clot, and uncontrolled bleeding then drives the patient further into shock, acidosis and cold. The goal of early trauma management is to prevent the cycle starting, or to break it quickly.

Cold is the element pre-hospital teams can influence most

A 2025 study in Injury analysed five years (2017 to 2021) of the US Trauma Quality Improvement Program database, covering 3,043,030 adult trauma patients. About 1% arrived hypothermic, defined as a temperature below 35°C.

Hypothermic patients were more severely injured, and their crude mortality was 23.4% against 2.3% for patients who arrived warm.
After adjustment, hypothermia was independently associated with about 1.9 times the odds of death (odds ratio 1.934). In patients with isolated traumatic brain injury the figure was 1.7 (odds ratio 1.728).
They also had more in-hospital complications (17.1% versus 4.5%) and longer stays in intensive care and hospital.
Patients who were rewarmed had lower mortality and shorter stays.

This is a retrospective study (level III evidence), so it shows association, not proof that rewarming saves lives. Hypothermic patients were also sicker, which the adjustment can only partly account for. But the direction of the finding fits the physiology and the long-standing teaching.

Why calcium joined the list

Calcium is an essential cofactor for clotting, platelet function, heart muscle contraction and the tone of blood vessels. Only the ionised fraction is biologically active, and the body normally holds it between about 1.1 and 1.3 mmol/L.

Several things push ionised calcium down in a bleeding patient:

Citrate in blood products. Stored blood contains citrate, which binds ionised calcium. The more blood given, and the faster, the greater the effect.
Shock and cold. Hypothermia and haemorrhagic shock reduce the liver’s ability to clear citrate, which compounds the problem.
Dilution. Other resuscitation fluids can lower calcium through haemodilution.
Injury itself. Low calcium is found in some patients on arrival, before any blood has been given.

A 2020 review in the Journal of Trauma and Acute Care Surgery asked whether the lethal triad should become the lethal diamond, and a 2021 article in the American Journal of Emergency Medicine called hypocalcaemia the "diamond of death". The idea has since been repeated widely.

How common is low calcium in trauma?

Estimates vary a great deal. A review cited in the German registry papers puts hypocalcaemia on arrival at the emergency department at between 13% and 74% of patients, depending on the study. In a large German registry cohort of 30,183 adult major trauma patients (TraumaRegister DGU, 2015 to 2019), 13.2% had ionised calcium below 1.10 mmol/L on arrival and 3.2% had calcium of 1.30 mmol/L or above. In a French registry cohort of patients who had been transfused, 64% developed hypocalcaemia within the first 24 hours. The difference reflects who was studied and when calcium was measured.

The relationship is not a straight line

The German registry analysis found that both low and high calcium were bad. Patients with hypocalcaemia and patients with hypercalcaemia were both more likely to have severe injury, shock, acidosis, coagulopathy, a transfusion requirement and haemorrhage as the cause of death, and both had lower survival. Hypercalcaemia on arrival was linked to the highest mortality. After adjustment:

Death within 6 hours was independently associated with ionised calcium below 0.90 mmol/L (odds ratio 2.69), with 1.30 to 1.39 mmol/L (odds ratio 1.56) and with 1.40 mmol/L or above (odds ratio 2.87).
Both calcium below 1.10 mmol/L and calcium of 1.30 mmol/L or above were independently associated with coagulopathy and transfusion.

The authors conclude that the relationship is parabolic, and that calcium may reflect how severe the injury and physiological derangement are, rather than being a separate problem that needs correcting in its own right. This is a key point. If a low calcium is mainly a marker of how sick the patient is, giving calcium may not change the outcome.

Is the diamond better than the triad?

A multicentre French analysis published in the World Journal of Emergency Surgery in 2025 compared the two directly. It used the TraumaBase registry from 2011 to 2023 and included 2,141 severe trauma patients who had received at least one unit of red cells in the first six hours and had an ionised calcium measurement. Median injury severity score was 27, and 24-hour mortality was 16.1%.

The lethal triad and the lethal diamond were associated with 24-hour mortality to a similar degree: area under the curve 0.72 for the triad and 0.71 for the diamond (p = 0.26).
All four elements were individually linked to mortality in the simple analysis, but after adjusting for the other three, hypocalcaemia was no longer independently associated with death (odds ratio 1.11, 95% confidence interval 0.84 to 1.48), while hypothermia, acidosis and coagulopathy each remained significant.
The diamond (all four elements) was present in 23.7% of patients, and 24-hour mortality in that group was 32.6%.

The authors conclude the evidence is not sufficient to treat hypocalcaemia as a fourth, equal element. They also argue that both constructs may oversimplify trauma haemorrhage, and that fibrinolysis, endothelial dysfunction and cellular energy failure also matter. Other studies have found an independent link between hypocalcaemia and early death, so the picture is mixed, and differences in when calcium was measured and whether it was given make direct comparison difficult.

Does giving calcium help?

The key question for practice is whether correcting calcium improves survival. The best large observational attempt so far is a propensity-matched analysis from the German registry, published in Critical Care in 2024.

Of 28,323 patients, 1,593 (5.6%) received calcium during initial resuscitation. They were much sicker: median injury severity score 33 against 20, and more were in shock, acidotic and coagulopathic.
After matching 1,447 patients who received calcium with 1,447 who did not, mortality at 6 hours was the same (5.7% in both groups) and the difference at 24 hours was not significant (14.6% with calcium against 12.5% without).
In-hospital mortality was 28.3% with calcium and 24.5% without, but the calcium group’s expected mortality was also higher. After accounting for predicted mortality the standardised mortality ratio was 1.02 against 0.95 (p = 0.244), so the difference was not significant.
About one in five patients who received calcium still had low calcium on arrival in intensive care, and hypercalcaemia was more frequent in the calcium group (14.5% against 9.2%).

The authors found no evidence for or against a survival benefit. The study could not tell the dose, timing or formulation of calcium, or why it was given, and it cannot rule out confounding. The data also come from 2015 to 2019, when pre-hospital transfusion was not routine. They note that a randomised trial of calcium in out-of-hospital cardiac arrest was stopped early with a trend towards harm, though cardiac arrest is a very different setting from trauma haemorrhage.

What is coming

CAVALIER is a randomised, double-blind trial of pre-hospital calcium and early in-hospital vasopressin in patients at risk of haemorrhagic shock. It is planned to enrol 1,050 patients across 13 US trauma centres under an exception from informed consent, and its primary outcome is 30-day mortality. I could not find published results.
A University of Maryland teaching summary describes a recent observational report in the Journal of Trauma and Acute Care Surgery linking at least 1 g of calcium chloride per 2 units of low-titre O whole blood with an 84% reduction in 24-hour mortality. It also notes that Joint Trauma System guidance recommends 1 g of calcium after the first unit of blood product and after every fourth unit. The 84% figure comes from observational data, which is open to the same confounding discussed above.

Until trial results arrive, major civilian guidelines generally advise preventing hypocalcaemia without much detail on timing or dose.

The studies side by side

Study	Setting and size	Main finding
Jose et al., Injury 2025	US TQIP database 2017 to 2021, 3,043,030 patients	1% hypothermic; crude mortality 23.4% vs 2.3%; adjusted odds ratio 1.93; rewarming linked to lower mortality
Helsloot et al., Critical Care 2023	TraumaRegister DGU, 30,183 patients	Hypocalcaemia 13.2%, hypercalcaemia 3.2%; U-shaped relationship with coagulopathy, transfusion and mortality
Dupuy et al., WJES 2025	French TraumaBase, 2,141 transfused patients	Triad AUC 0.72 vs diamond 0.71 (p = 0.26); hypocalcaemia not independently linked to 24-hour mortality (odds ratio 1.11)
Helsloot et al., Critical Care 2024	TraumaRegister DGU, 1,447 matched pairs	No difference in early mortality; in-hospital mortality 28.3% vs 24.5%, not significant after adjustment (SMR 1.02 vs 0.95)

What these studies cannot tell us

All four studies in the table are retrospective and observational, so they show association, not cause.
Calcium measured on arrival at hospital says little about calcium in the field, and none of these four studies measured it before hospital.
The calcium studies could not account for why, when or how much calcium was given.
Thresholds, timing of measurement and definitions of hypocalcaemia differ across studies, which is one reason their conclusions differ.
The hypothermia data are US figures, and the registry data are mainly German and French. None of this describes the UAE or the UK directly.

What this means for pre-hospital care

Calcium and blood products are clinician-level treatments. For everyone else involved in the early phase, the evidence points to some familiar priorities:

Stop the bleeding first. Every element of the triad and the diamond gets worse as blood loss continues. Haemorrhage control sits at the front of DCR-ABCDE for that reason.
Protect against heat loss from the start. Cold is the element that rescuers can influence most directly, and rewarming was linked to better outcomes in the large US analysis, though that evidence is retrospective. An older single-centre audit, reported as a conference poster in 2014, found only 24% of trauma patients had a pre-hospital temperature recorded.
Remember that fluids and blood are not neutral. The registry authors note that blood products and other resuscitation fluids can both lower ionised calcium.
Hand over what was given. Time of injury, interventions, fluids and any blood products help the receiving team decide what needs correcting next.

Echelon’s training is built around UK pre-hospital care frameworks.

Key takeaways

Admission hypothermia was linked to about double the odds of death in a database of over 3 million US trauma patients, and rewarming was linked to better outcomes.
Low calcium is common in severe trauma and tied to coagulopathy and transfusion, but the relationship with outcome is U-shaped, with high calcium also linked to harm.
In a French cohort of transfused patients, the lethal diamond predicted early death no better than the lethal triad, and hypocalcaemia lost its independent link to death after adjustment.
A large matched cohort found no evidence for or against a survival benefit from calcium during trauma resuscitation. The CAVALIER trial should give a clearer answer on pre-hospital calcium.

Over to you: Does the diamond deserve to replace the triad, or is it too early to say?

Sources

Interested in pre-hospital trauma training for your team? Contact operations@echelonmedicalsolutions.com.