The goal of this observational study is to learn whether there are age- and sex-specific threshold levels of blood lactate that can better predict the risk of death and serious illness in people receiving intensive care, especially those with sepsis (a severe infection that affects the whole body).

Lactate is a chemical that the body produces during metabolism. High levels in the blood are often a sign that tissues are not getting enough oxygen, but newer research shows that lactate may also rise due to stress hormones and changes in how cells use energy.

Doctors currently define septic shock as sepsis with low blood pressure that does not improve after fluids, combined with a lactate level higher than 2 millimoles per liter (mmol/L). This value was set based on expert agreement but may not be ideal for everyone. Recent studies suggest that even lactate levels below 2 mmol/L can still be linked to a higher chance of dying in the hospital.

This study aims to find out if different cutoff levels for men and women, and for different age groups, could improve how doctors identify patients at higher risk, compared with using the same general value for everyone.

Main questions:

Do older adults and younger adults have different blood lactate thresholds linked to worse outcomes in sepsis?

Do men and women show different relationships between lactate levels and risk of death or complications?

Can combining lactate levels with other factors (such as blood pressure or medical history) improve predictions of outcome in intensive care?

Study design:

This is a retrospective observational study based on data from Uppsala University Hospital in Sweden. The study includes all people admitted between 2016 and 2024 who had blood lactate measured during their hospital stay. People without lactate measurements or with unknown identity are excluded.

Researchers will analyze the relationship between lactate levels, age, sex, and survival in intensive care. They will use statistical models to find threshold values of lactate linked to higher risk of death or need for advanced care. Machine learning methods, such as clustering algorithms, will be used to identify patient subgroups with similar biological patterns. The researchers will also perform sensitivity analyses to test whether their findings are robust.

Why this study matters:

If reliable age- and sex-specific lactate thresholds can be identified, doctors may be able to detect patients at risk earlier, even when lactate levels seem normal. This could help guide treatment and monitoring more precisely for each person. The results may contribute to a more personalized definition of septic shock and influence future international guidelines for sepsis management.

Background:

Traditionally, high lactate levels in sepsis were believed to mean tissues were not getting enough oxygen. However, new evidence shows that lactate can also increase for other reasons, such as overactive stress responses, even when oxygen levels are normal. The liver and kidneys usually remove lactate from the blood, but their function may be reduced during severe infection. Treatments for septic shock currently focus mainly on maintaining blood pressure, but this does not always reflect how well oxygen reaches tissues. Current definitions do not consider differences in how people of various ages or sexes produce or clear lactate. Understanding these differences could improve how doctors interpret blood tests and adjust treatments.

Potential benefits:

The study will not directly involve new treatments, since it uses existing hospital data. However, its findings may help improve early detection of severe infection, support development of personalized treatment strategies, and reduce death rates among people with sepsis in intensive care.

In summary, this study seeks to find better ways to interpret blood lactate levels in intensive care by focusing on age and sex differences. The results may lead to more accurate risk prediction and individualized care for people with sepsis.

Inclusion Criteria:

• Having any recorded lactate

Exclusion Criteria:

• lacking lactate

Any patient where lactate was analysed at Uppsala University Hospital in Sweden between 2016 and 2024

Data can be shared with other researchers upon request after appropriate ethical permissions and data transfer agreements.

2035

Researchers must have ethical permission and fullfil Swedish and European Data Safety Standards for sensitive health data and obtain appropriate Data Transfer Agreements.