Journal of Informatics in Primary Care 1997 (November):8-12

Objective: to assess the concurrent validity of the International Classification of Primary Care (ICPC) in the classification of patient reasons for encounter. Design: Analyses of utilisation of ICPC codes in classifying patient reasons for encounter (RFEs). Setting: primary health care. Subjects: 146,940 patient RFEs recorded by general practitioners in an Australian National survey. Main outcome measures: Relative frequency of utilisation, classed as: frequent (>5/1,000 contacts); intermediate (1–5/1,000); marginal (0.5–1/1,000); or rare(<0.5/1,000 contacts). Relative use of ‘rag-bag’ codes which group multiple concepts. Results: Of 1,371 available codes 76.7% were used at least once. Over two-thirds of all RFEs were classified in Components 1 (symptoms/complaints) or 7 (diagnoses/disease), in which 93.8% of rubrics were selected. Only 48 ICPC codes accounted for 65.5% of all RFEs. Codes never used totalled 319, only 43 of these being in Components 1 and 7. Only 3.3% of RFEs were classified in the 86 identified ‘rag-bag’ codes. In relative terms, the proportion of RFEs associated with the eye (19.6%) and the male genital system (17.6%) coded in ‘rag-bags’ was high. Conclusion: ICPC was found to be a valid tool with which to classify RFEs, but some suggestions for improving future versions of the classification are put forward.

There has been one major study of patient reasons for consulting a general practitioner (GP). The Australian Morbidity and Treatment Survey (AMTS) 1990–1991 was a cross-sectional study in which a random sample of 495 GPs actively recorded details about more than 100,000 doctor–patient contacts on structured paper-based contact forms. Variables included patient demographics, patient reasons for encounter (RFEs), problems managed and treatments provided. At least one and up to three RFEs were requested for each contact[1]. The 146,940 patient RFEs recorded were secondarily coded with the International Classification of Primary Care (ICPC)[2].

ICPC is also being used to classify RFEs and/or problems managed in Norway[3], Canada[4,5], the Netherlands[6,7,8], Belgium[9] and the United States[10]. The extent to which ICPC is a valid tool with which to classify such data should therefore be assessed.

Previous work has reported: the extent to which the GP and patient samples represent all GPs and all patients attending any general practitioner[11]; the degree to which patient RFEs perceived and recorded by the GP accurately reflect patient recalled RFEs[12]; and the reliability of secondary coding of the recorded RFEs[13]. A related paper investigated inter-practitioner reliability and validity of the recorded morbidity[14].

This paper addresses the next question: whether ICPC is a valid tool with which to classify the data. Application of an inappropriate classification, just as application of unsuitable statistical methods, can render results drawn from a valid dataset invalid. For example, ICD and the Canadian General Practice Classification are unsuitable tools for classifying primary care data because they lack rubrics for many problems commonly managed in general practice[15,16].

Validity of an instrument is usually measured by comparing results with a ‘gold standard’ set by another instrument. This approach is often referred to as ‘convergent’ or ‘criterion’ validity. Where no gold standard is available, validation can only occur "by the gradual accumulation of data from many different kinds of investigations"[17].

ICPC was designed and field-tested by a group of experts in classification of general practice data[18], and so could be said to have face validity. It could also be said to have ‘consensual validity’ since its structure and rubrics were chosen through consensus of these experts[19]. However, these types of validity are biased by the subjective opinions of the experts and further assessment of the validity of ICPC is warranted. There is no gold standard against which results using ICPC can be compared, but we can begin to assess the concurrent validity of ICPC in its RFE mode, using the AMTS data. Kidder and Judd[20] define concurrent validity as "the ability of a test (or instrument) to distinguish between people who are known to differ". Messick[21] suggests it would be better described as "diagnostic utility". This paper explores several aspects of the validity of ICPC as a tool to classify patient RFEs in a manner that reflects the breadth of available information.

ICPC is a hierarchical classification, which only provides a specific code for the more common or more important problems dealt with in primary care. Any classification must also include some unspecific codes to cover less common conditions and poorly described information not classified elsewhere. Wood et al[22] refer to these as ‘rag-bag codes’. However, if rubrics are poorly selected and do not satisfactorily describe the concept, or describe problems rarely seen in primary care, many may never be used. If too large a proportion of concepts are classified in ‘rag-bag’ codes, the classification has failed to identify some concepts which should have their own rubrics. Likewise, if a large proportion fall into few individual rubrics and the remainder are lumped into the ‘rag-bag’ codes, many rubrics would remain unused and the classification would not discriminate sufficiently between individual concepts.

Studies investigating utilisation of the codes available in a classification are rare. As part of a broader study of the introduction of ICPC in Norway, Brage et al[3] included a brief overview of utilisation rates in coding morbidity for sickness certification. Lamberts et al[22] compared code utilisation in the classification of morbidity and, in some cases RFEs, from a number of studies in which data were either already coded in ICPC or were mapped to it. Results were reported for 91 ‘rag-bag’ codes but the codes included were not identified. Results were reported in terms of rate per 1,000 patient years, rendering them non-comparable with those of the present study, which is consultation-based. However Lamberts’ utilisation classes have been adapted for the following analysis, results being reported in terms of rate per 1,000 contacts.

Table 1: Utilisation of ICPC component in classifying RFEs and diagnoses in the AMTS

Table 2: Utilisation rates of codes from ICPC Component 1 (symptoms and complaints) and Component 7 (diagnosis/disease) (N=691) in the classification of patient reasons for encounter – rate per 1,000 contacts

Table 3: Distribution of ‘rag-bag’ utilisation in the classification of patient reasons for contact by ICPC chapter

The very small proportion of total RFEs that were classed in the ‘rag-bag’ codes suggests that ICPC provides sufficient individual rubrics to cover the large majority of patient RFEs described in general practice. Codes -29 and -99 accounted for only 1.5% of total RFEs. In contrast, in the Norwegian Sickness Benefit Register, where the practitioners coded data, 8.9% of all diagnoses were coded in this manner[3]. One would anticipate that secondary coding of patient RFEs would have resulted in higher relative use of ‘rag-bags’. Firstly the patient description of the reason for contact is likely to be less specific than the doctor’s problem label/diagnosis. Secondly, if the clinician is coding s/he has greater knowledge of the problem than is available to secondary coders working with paper records. The differences in results may be due to the use of an expanded and highly detailed index in the present study[13] that led the coders to the correct rubric. In Brage’s study[3], where a problem label was not listed in the index, practitioners may have tended to choose ‘other symptoms’ and ‘other diagnoses’ codes. While the data being classified are different in the two studies the Norwegian study also noted the high proportion of ‘rag-bag’ allocation in the eye chapter and this suggests that the rubrics available in this chapter should be critically evaluated.

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