From the Division of Neurology, Department of Medicine, the Duke Center
for Cerebrovascular Disease, and the Center for Clinical Health Policy
Research, Duke University, Durham, NC, and the Division of Neurology, Durham
Department of Veterans Affairs Medical Center, Durham, NC.
Correspondence to Larry B. Goldstein, MD, Box 3651, Duke University Medical Center, Durham, NC 27710. E-mail golds004{at}mc.duke.edu
Methods—Available hospital charts for all patients discharged
from a single hospital between May 1995 and June 1997 with ICD-9-CM
codes 433 (occlusion and stenosis of precerebral arteries), 434
(occlusion of cerebral arteries), or 436 (acute but ill-defined
cerebrovascular disease) listed in the first position were reviewed.
The primary discharge diagnosis was verified, and a presumed stroke
subtype was assigned on the basis of information provided in the
medical record.
Results—Charts were available for 175 of the 198 identified
patients (88%). Of these, 61% had an acute ischemic stroke
(code 433, 4%; 434, 82%; 436, 79%) with the remaining patients
having other conditions. Of the 130 patients with a modifier code
indicating cerebral infarction, 79% had an acute stroke; of the 45
patients with a modifier code indicating an absence of cerebral
infarction, 7% had acute stroke (sensitivity, 0.97; specificity,
0.60). The codes with the highest proportions of ischemic
stroke cases were 434.11 (embolic occlusion of cerebral arteries with
infarction, 85%), 434.91 (unspecified occlusion of precerebral
arteries with infarction, 82%), and 436 (79%), with a combined
sensitivity of 0.81 and specificity of 0.90. On review, 73% of
patients with code 434.11 had embolic strokes, and 47% of those with
code 436 had an identified stroke cause. Of patients with code 434.91,
39% had stroke of uncertain cause, 25% "lacunar," 17%
atherothrombosis, and 15% embolism.
Conclusions—Despite the use of modifier codes, 15% to 20% of
patients with the indicated primary ICD-9-CM codes have conditions
other than acute ischemic stroke. Although the proportion of
patients with acute stroke increased from 61% to 79% with the use of
modifier codes, the inclusion of modifier codes did not have an
appreciable effect on the accuracy of the coding if patients with code
433 are excluded. Assignment of presumed ischemic stroke
subtype is particularly inaccurate.
The aim of this study was to determine if the accuracy of the primary
ICD-9-CM codes for the identification of incident ischemic
stroke is improved by the use of modifier codes (43X.X1, with
infarction; 43X.X0, without infarction, Table 1
Figure 1
Of the 130 patients with a modifier code indicating cerebral infarction
(code 436 was not used with a modifier and was considered as acute
stroke), 79% had an acute stroke; of the 45 patients with a modifier
code indicating an absence of cerebral infarction, 7% had acute stroke
(sensitivity, 0.97; specificity, 0.60). For the 43 patients given code
433.X0, 42 were correctly classified as not having stroke (Figure 1
Stroke subtype diagnoses are also included in the ICD-9-CM coding.
Figure 2
The use of modifier codes as an indicator of stroke subtype was
particularly inaccurate (Figure 2
One limitation of the present study is that it is based on patients
admitted to a single Veterans Administration Medical Center. However, a
consecutive series of patients were included and the overall accuracy
of the primary ICD-9-CM codes was virtually identical to that found in
large population-based samples.3 4 5 Therefore,
the results are likely generalizable to other populations of
patients.
Based on these results, if a study is designed to follow trends in
costs and patterns of care for patients with acute ischemic
stroke, it is reasonable to exclude those with a discharge code 433 and
to include patients with codes 434 and 436, regardless of a modifier
code indicating the presence or absence of cerebral infarction. This
can be accomplished without discharge summary review only if an error
rate of 15% to 20% is understood and deemed acceptable. Review of the
patients' medical records would be necessary if greater degrees of
accuracy are required or if complete case ascertainment is necessary.
The modifier codes cannot be used to assign stroke subtypes.
Received March 18, 1998;
revision received May 8, 1998;
accepted May 8, 1998.
© 1998 American Heart Association, Inc.
Original Contributions
Accuracy of ICD-9-CM Coding for the Identification of Patients With Acute Ischemic Stroke
Effect of Modifier Codes
Abstract
Top
Abstract
Introduction
Subjects and Methods
Results
Discussion
References
Background and Purpose—Discharge
ICD-9-CM (International Classification of Diseases, 9th
Revision, Clinical Modification) codes have been used to
identify patients with acute stroke for epidemiological, quality of
care, and cost studies. The aim of this study was to determine if the
accuracy of the primary ICD-9-CM codes for ischemic stroke is
improved by modifier codes and how specific codes reflect stroke
subtype diagnoses.
Key Words: classification • epidemiology • stroke, ischemic • diagnosis
Introduction
Top
Abstract
Introduction
Subjects and Methods
Results
Discussion
References
Computerized
databases such as Medicare claims files and hospital administrative
records are increasingly being used to identify patients with acute
stroke for epidemiological, quality of care, and cost
studies.1 2 3 These databases rely on patient
diagnoses as classified according to the International
Classification of Diseases, 9th Revision, Clinical
Modification (ICD-9-CM). However, the accuracy of ICD-9-CM coding
for the identification of incident strokes can be poor. For example,
separate community-based studies in Rochester, Minnesota, and
Cincinnati, Ohio, found that only 46% to 47% of patients with a
primary diagnosis code of 430 to 438 (Table 1
) had an incident
stroke.3 4 Other investigators have found that
limiting the identifying ICD-9-CM code to the primary position in
administrative databases increased the proportion of patients
accurately classified as having a stroke on subsequent review of their
medical records.5 In agreement with other
work, this study also found that the accuracy of the ICD-9-CM
classification depended on the specific code that was used. Only 15%
of patients with code 433 were symptomatic for the index
hospital admission. In contrast, 85% of patients with code 434 and
77% of those with code 436 had ischemic strokes. Based on
these data, it is apparent that if a study intends to identify all
patients in a given setting with an acute stroke based on ICD-9-CM
codes, discharge summary review would be required to eliminate the high
proportion of nonstroke cases. However, if the goal of a study is to
follow trends or patterns of care, then identification of
subpopulations with a high likelihood of having stroke would be
advantageous because doing so could eliminate the need for extensive
review of patients' medical records.
View this table:
[in a new window]
Table 1. ICD-9-CM Stroke
Codes 
, "X" indicates an
integer code number) and how specific modifier codes reflect stroke
subtype diagnoses.
Subjects and Methods
Top
Abstract
Introduction
Subjects and Methods
Results
Discussion
References
Available hospital charts for all patients discharged from the
Durham Veterans Affairs Medical Center between May 1995 and June 1997
with ICD-9-CM codes 433, 434, and 436 listed in the primary position
were reviewed by the investigator (n=175 of 198, 88%). These were
chosen based on previous studies indicating that the majority of
patients with ischemic stroke are classified with these
codes.3 4 5 Selection was limited to the primary
position codes to increase the likelihood of identifying incident
rather than prevalent strokes. The primary discharge diagnosis was
verified by review of information provided solely in the discharge
summary (available for each reviewed record), and a presumed stroke
subtype was assigned by the investigator into 1 of 5 categories based
on criteria developed for the TOAST (Trial of ORG 10172 in Acute Stroke
Treatment): large artery atherothrombosis (atherothrombotic),
cardioembolism (embolism), small-vessel occlusion,
other determined etiology, or uncertain
etiology.6 The sensitivities and specificities of
each code were then calculated.
Results
Top
Abstract
Introduction
Subjects and Methods
Results
Discussion
References
Of the 175 reviewed cases, 61% (n=106) had an acute
ischemic stroke, and the remaining patients (n=69, 39%) had
other conditions. The most common other reasons for hospitalization are
given in Table 2. Over 50% of the
patients with conditions other than ischemic stroke were
hospitalized for cerebral angiography or carotid
endarterectomy. Miscellaneous reasons for
hospitalization (1 case each) included hypertension, dementia,
arteritis, cardiac arrest, encephalopathy, atrial fibrillation, and
nausea of uncertain etiology.
View this table:
[in a new window]
Table 2. Nonstroke Diagnoses With Discharge Codes 433, 434,
and
436 gives the proportions of
incident stroke cases under each diagnostic code including
the modifier codes (43X.X1, with infarction; 43X.X0, without
infarction). Only a small proportion of the patients with discharge
codes 433.XX had an incident stroke, whereas approximately 80% of
patients given discharge codes of 434.XX or 436 had new
ischemic strokes (code 433, 4%; 434, 82%; 436, 79%).
Together, these latter 2 codes accounted for 98% of the identified
cases of incident ischemic stroke. The codes with the highest
proportions of incident ischemic stroke cases were 434.11
(85%), 434.91 (82%), and 436 (79%), with a combined sensitivity of
0.81 and specificity of 0.90. These codes accounted for 101 of the 106
patients (95%) found to have ischemic stroke on subsequent
discharge summary review.
View larger version (34K):
[in a new window]
Figure 1. The proportions of patients with each ICD-9-CM
discharge code who had had an acute stroke based on review of their
medical records are given (code 433.X0, occlusion and
stenosis of precerebral arteries without infarction; 433.X1,
occlusion and stenosis of precerebral arteries with infarction;
434.X0, occlusion of cerebral arteries without infarction; 434.X1,
occlusion of cerebral arteries with infarction; and 436, acute but
ill-defined cerebrovascular disease). ).
In contrast, only 1 of 5 patients with code 433.X1 was correctly
classified as having acute stroke. The majority of patients given code
434 had acute stroke regardless of the modifier code. The 19 patients
with code 434 not having stroke were incorrectly given code 434.X1
(indicating the presence of acute stroke). Of patients given code
434.X0 (modifier code indicating an absence of cerebral infarction),
100% (2 patients) had an acute stroke. gives the diagnoses based on
discharge summary review for each of these specific discharge
diagnostic codes. Of patients discharged with a code
indicating cerebral embolism with infarction (434.11), 27% had
nonembolic strokes. Of patients discharged with a code indicating an
unspecified occlusion with infarction (434.91), 60% had an
identifiable cause of their strokes on review of their hospital
records (15% had embolic, 17% atherothrombotic, and 25%
small-vessel distribution strokes). Similarly, 47% of those with code
436 (acute ill-defined cerebrovascular disease) had an identified
stroke cause (8% had embolic, 12% atherothrombotic, and 27%
small-vessel distribution strokes).
View larger version (26K):
[in a new window]
Figure 2. The final diagnoses (based on TOAST6
criteria; embolism, cardioembolic strokes; atherothrombotic, large
artery atherosclerosis; small vessel, small-vessel
occlusion; uncertain, stroke of undetermined etiology) following
discharge summary review are given for each of the indicated ICD-9-CM
discharge codes (434.11, embolic occlusion of cerebral arteries with
infarction; 434.91, unspecified occlusion of precerebral arteries with
infarction; 436, acute but ill-defined cerebrovascular disease). There
were no patients classified as having a stroke of other determined
etiology.
Discussion
Top
Abstract
Introduction
Subjects and Methods
Results
Discussion
References
Despite the use of modifier codes, approximately 15% to 20% of
patients with the indicated primary ICD-9-CM discharge codes had
conditions other than acute ischemic stroke. As has been
reported previously,5 the highest proportion of
nonstroke diagnoses had discharge code 433, and less than 2% of these
patients had an acute stroke. Although the majority of these patients
were given code 433.X0 indicating occlusion or stenosis of the
precerebral arteries without infarction, only 1 patient with acute
stroke was given code 433.X1. Of those given a code of 434.X1
indicating the presence of cerebral infarction, 18% did not have
incident strokes. Therefore, although the proportion of patients with
acute stroke increased from 61% to 79% with the use of modifier codes
(4XX.X1), a strategy for identifying patients with acute
ischemic stroke using modifier codes has a yield similar to one
in which all patients with code 433 are excluded and all patients with
codes 434 and 436 are included. If this approach had been used in the
present study, discharge summary review could have been reduced by
over 25% with a loss of only 2 incident stroke cases (1.8%). ). About 25% of patients given a code
indicating embolic cerebral infarction had other causes of their
strokes identified on review of their medical records.
Approximately 50% to 60% of patients given codes indicating
unspecified causes for stroke had an etiology established during the
hospitalization as reflected in their discharge summaries. Although
discharge coding is not typically done by neurologists (or physicians
in general), accuracy may not necessarily be improved with further
training of the coders. One study presented a series of case
scenarios to a group of neurologists who were asked to assign specific
discharge ICD-9 codes (which included a variety of modifier
codes).7 The chance corrected interobserver
reliability of the classifications (kappa score) was 0.38, indicating
only fair levels of agreement among the
raters.8
References
Top
Abstract
Introduction
Subjects and Methods
Results
Discussion
References
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