Severity-of-illness Scoring Systems in ICU:

The scoring systems most commonly used in critically ill
adults are APACHE II, APACHE III, MPM II, SAPS II, and SOFA.

Calculation of Acute Physiology and Chronic Health Evaluation II (Apache II)

The Acute Physiology and Chronic Health Evaluation II
(APACHE II) system is the most commonly used clinical
severity-of-illness scoring system in North America. APACHE II is a disease-specific scoring system. It uses age, type of admission, chronic health evaluation, and 12 physiologic
variables (acute physiology score or APS) to predict hospital mortality. The 12 physiologic variables are defined as the most abnormal values during the 24 hours after ICU admission.

Acute Physiology Score: 

APACHE III:

APACHE III is a disease-specific score that was developed
from 17,440 admissions in 40 U.S. hospitals. Eighteen
variables were included, and their respective weights were derived by logistic regression modeling.

The final APACHEIII score can vary between 0 and 300.
Risk estimate equations for hospital mortality are calculated
from the weighted sum of disease category (78 diagnostic categories are included), a coefficient related to prior treatment
location, and the APACHE III score. In the original derivation
sample, estimates of mortality for the first day in the ICU
had an area under the ROC curve of 0.90, and the correct
classification at 50% mortality risk level was 88%. Although
APACHEIII scores can be calculated from published information, weights to convert the score to probability of death are proprietary, therefore the APACHE III system has not been
widely accepted or used.

 MPM II:

The Mortality Probability Model(MPMII)20 was developed from 19,124 ICU admissions in 12 countries. MPM II is not disease specific. MPM-0 is the only severity-of-illness scoring system that was derived at ICU admission and can therefore be used at ICU admission. MPM II does not yield a score, but rather a direct probability of survival. Burn, coronary care, and cardiac surgery patients are excluded. MPM-0 includes three physiologic variables, three chronic diagnoses, five acute diagnoses, and three other variables: cardiopulmonary resuscitation prior to admission, mechanical ventilation, and medical or unscheduled surgery admission. Each variable is scored as absent or present and is allocated a coefficient. The sum of these coefficients constitutes the logit that is used to calculate the probability of hospital mortality.

The MPM-24 was designed to be calculated for patients who remained in the ICU for 24 hours or longer. MPM-24 includes 13 variables, 5 of which are used in the MPM-0.  

SAPS II:

The most recent version of the Simplified Acute Physiology Score II (SAPS II) was developed from a sample of 13,152 admissions from 12 countries, based on a European/North American multicenter database. SAPS II is not disease specific.
SAPS II uses 17 variables that were selected by logistic regression: 12 physiology variables, age, type of admission (scheduled surgical, unscheduled surgical, or medical), and three underlying disease variables (acquired immunodeficiency syndrome, metastatic cancer, and hematologic malignancy).  The probability of hospital mortality is calculated from the score

 

Type of Admission	Scheduled surgery	0
	Unscheduled surgery	8
	Medical	6
Chronic diseases	None	0
	Metastatic Carcinoma	9
	Hematological malignancy	10
	AIDS	17
Age	<40	0
	40-59	7
	60-69	12
	70-74	15
	75-79	16
	≥ 80	18
Temperature	<39	0
	>39	3
Heart Rate	<40	11
	40-69	2
	70-119	0
	120-159	4
	≥160	7
Systolic Blood Pressure, mmHg	<70	13
	70-99	5
	100-199	0
	≥200	2
Urine output, cc/24 hrs	<500	11
	500-999	4
	>1000	0
Glasgow Coma score	<6	26
	6-8	13
	9-10	7
	11-13	5
	14-15	0
Serum Urea or BUN	<10	0
	10-29.9	6
	≥30	10
Serum sodium, mEq/L	>146	1
	125-144	0
	<125	5
Serum Potassium, mEq/L	<3	3
	3-4.9	0
	>5	3
WBC, per cubic mm	<1000	12
	1000-19000	0
	>20000	3
Bicarbonate, mEq/L	<15	6
	15-19	3
	>20	0
Serum Bilirubin, mg/dl	<4	0
	4-5.9	4
	>6	9
PaO2/FiO2, (if ventilated or CPAP)	<100	11
	100-199	9
	>200	6

SOFA Score:

The Sequential Organ Failure Assessment (SOFA) was originally developed as a descriptor of a continuum of organ dysfunction in critically ill patients over the course of their ICU
stay.The SOFA score is composed of scores from six organ systems, graded from 0 to 4 according to the degree of dysfunction/failure. The score was primarily designed to describe morbidity; however, a retrospective analysis of the relationship between the SOFA score and mortality was developed using the European/North American Study of Severity System database. Subsequently, SOFA was evaluated as a predictor of outcome in a prospective Belgium study. SOFA score on admission was not a good predictor of mortality; however, mean SOFA score and highest SOFA score had better discrimination. Independent of the initial value, an increase in the SOFA score during the first 48 hours of ICU admission predicts a mortality rate of at least 50%.