Westgard System and Rules
Is 1
Is 1
control
control
Y
es
> 2 SD ?
> 3 SD ? N o


12 S
No
13 S
Y es
A ccept
run
Reject
run
Report
Results
Test
rem aining
rules
V iolation
indicates
random
error
Where:
A re 2
controls
> 2 SD N o
on same 
side of
mean?
Is SD
difference
No
betwe en 
any 2
controls
> 4?
22 S
Y es
R4 S
Y es
Reject
run
Reject
run
Test
rem aining
rules
Test
rem aining
rules
V iolation
indicates
system atic
error
V iolation
indicates
random
error
A re 4
consecutive
No
controls
> +/- 1 SD ? 
42 S
Y es
Reject
run
Test
rem aining
rules
V iolation
indicates
system atic
error
A re 10
consecutive
No
controls
on same 
side of
mean?
10X
A ccept
run
Report
Results
Y es
Reject
run
V iolation
indicates
system atic
error
12S = one value outside 2 SD, but < 3 SD
13S = one value outside 3 SD
22S = two values consecutively on same side of the mean, outside 2 SD, but < 3 SD
R4S = range or difference between two control values in same run exceeds 4 SD
41S = four consecutive values on one side of the mean more than 1 SD
10X = ten consecutive values on one side of the mean
BASIC RULES
Looking at the chart and following it through, the first step in identifying an out-of-control situation is to
see whether any control (regardless of how many are run) is outside the 2 SD range (12S).
1.
2.
3.
If no values are outside the 2 SD limits, the patient results are reported.
If one control value is outside the 2 SD, then following the arrow, the next step is to see
whether a value is outside 3 SD (13S). The value outside 3 SD could be the same control
that is outside 2 SD. Any time a value is outside 3 SD, the system should be examined for
random error; on other words, STOP.
If no value is outside 3 SD, then look to see whether two values are outside 2 SD (22S).
The 22S rule generally considers data points, entered consecutively.
For example, three levels of control are used with means and SDs of 100 ± 5,
150 ± 8, and 200 ± 10.
In Run A, the results are 111, 267, and 202. This would break the 22S rule (levels I and II
are outside + 2 SD).
In Run B, the results are 80, 152, and 175. This would not break the 22S rule (no 2
consecutive levels outside). In running three controls, one would consider the run “out”
if controls 1 and 2 are out, or controls 2 and 3 are out (22S). A third possibility, controls 1
and 3 are > 2 SD limit, might be considered “out-of-control”. In this case, the (2 of 3)2S
rule has been broken. If these rules have been violated, the system is examined for
systematic error.
4.
5.
6.
7.
If not, look at the distance from the mean of each control (number of SDs) in the run.
Should one be outside 2 SD and the algebraic difference between them is at least 4 SD,
STOP. This is a violation of the R4S rule.
If no violation at this point occurs, the system is then examined for four consecutive
values on one side of the mean and further than 1 SD from the mean (41S).
If this rule is not violated, the system is checked for 10 consecutive values on one side of
the mean (10X).
If this rule is not violated, all that we have is the single value outside 2 SD (but within 3
SD), as well as across control materials (all levels are evaluated together). The patient
results can be reported.