The Physiologic Stability Index (PSI) measures severity of illness, while the Therapeutic Intervention Scoring System (TISS) measures quantity of care. The ratio of the two relates the level of physiologic instability to amount of therapy required. Analysis of the ratio over time can show differences not apparent by analysis of the PSI or TISS alone. The ratio can be used to evaluate pediatric intensive care units.


The patients studied were classified by the Clinical Classification System:

(1) Class III (physiologically stable, requires intensive nursing and monitoring)

(2) Class IV (physiologically unstable, requires intensive nursing and physician care, with frequent patient assessment and therapy adjustment)


Admission service:

(1) medicine

(2) cardiovascular surgery

(3) non-cardiovascular surgery


PSI/TISS ratio =

= (PSI) / (TISS)



• Survivors tend to have lower ratios than nonsurvivors.

• Patients on the different services differ in the ratios seen, with highest ratios seen on medical services and lowest on the cardiovascular surgery service

PSI/TISS ratio at admission




0.40 +/- 0.03

0.57 +/- 0.05

cardiovascular surgery

0.18 +/- 0.01

0.33 +/- 0.05

other surgery

0.33 +/- 0.04

0.52 +/- 0.10


When the ratio is plotted over time, the slope of the line correlates with survival or nonsurvival.


Trend analysis over time




negative slope

zero or positive slope (except cardiovascular surgery)


negative slope

zero or positive slope

PSI/TISS, medicine

zero slope

zero slope

PSI/TISS, cardiovascular surgery

positive slope

negative slope

PSI/TISS, non-cardiovascular surgery

negative slope

positive slope


To read more or access our algorithms and calculators, please log in or register.