The International Classification of Diseases, 9th Revision, Clinical Modification (ICD) is a standard vocabulary for diagnoses, symptoms, and syndromes (see Chapter 32). ICD has a code for each class of diagnoses, syndromes, and symptoms that it covers. For example, the ICD code 034.0 Streptococcal sore throat includes tonsillitis, pharyngitis, and laryngitis caused by any species of Streptococcus bacteria. There are more than
12,000 ICD codes. Internationally, some countries use the 10th revision of the International Classification of Diseases, or a modification of it.
Data encoded using ICD are widely available in the United States. Most visits to physicians or other healthcare providers and hospitalizations result in one or more ICD codes. The reason is that healthcare insurance corporations require providers of care to use ICD codes when submitting insurance claims to receive reimbursement for their services.
ICD codes range in diagnostic precision from the very imprecise level of symptoms to very precise diagnoses. There are precise codes for infectious diseases, specifying both the causative organism and the disease process (e.g., 481 Pneumococcal pneumonia). However, there are less precise codes that providers can use if the organism is unknown or not documented (e.g., 486 Pneumonia, organism unspecified). There are also ICD codes for syndromes (e.g., 079.99 is the code for Viral syndrome) and even for symptoms (e.g., 786.2 Cough and 780.6 Fever).
ICD codes may be assigned at different times during the course of care (Figure 23.1). As you go from left to right in Figure 23.1, who assigns the ICD code, how, and when vary. Physicians, when they do assign ICD codes to office or ED visits, usually do so during or within hours to days of the visit. They either enter ICD codes into a point-of-care system or record them on an encounter form (also sometimes known as a "superbill''). Professional coders usually assign the final, billing ICD codes for ED and office visits days later. They also assign ICD codes to hospital discharge diagnoses, typically days to weeks after the patient leaves the hospital. Professional coders often enter ICD codes into specialized billing software. ICD-coded data from organizations that collect large volumes of insurance claims data (we discuss these "data aggregators'' in more detail below) are usually not available for months after visits or hospital stays.
The diagnostic precision of ICD codes generally increases with time, as you go from left to right in Figure 23.1.The reason that discharge diagnoses generally have higher diagnostic precision relative to visit diagnoses is that discharge diagnoses typically represent the outcome of a greater amount of diagnostic testing that leads to greater diagnostic certainty (i.e., providers are more likely to order—and have the results available from— laboratory tests, microbiology cultures, x-rays, and so on).
Health services researchers have established that the accuracy of ICD-coded data is highly variable and often only moderately high (O'Malley K et al., 2005, Hsia et al., 1988). They have identified several causes for inaccuracy (Peabody et al., 2004, O'Malley et al., 2005). One cause is that two different, highly trained, experienced coders may assign different codes to the same hospitalization (Fisher et al., 1992, Lloyd and Rissing, 1985, MacIntyre et al., 1997). One reason is that coders work from the patient chart, which is an imperfect representation of the patient's true medical history and is subject to variable interpretation. Professional coders are typically not physicians or nurses, so their level of understanding of the medical process is imperfect. Finally, the rules for assigning codes are complex and change at least annually.7
The problem of correct assignment of ICD codes is compounded when clinicians encode the diagnoses (Yao et al., 1999). Clinicians rarely have formal training in the rules for assigning codes. They typically have little time to ensure that the codes they assign are accurate. They often view the assignment of ICD codes to patient encounters as a distraction from patient care. To address these problems, physicians often use preprinted encounter forms that have check boxes for an extremely small subset of commonly used codes. Although these forms typically include a blank space to write in additional ICD codes, clinicians are extremely busy so an open question is how often they use the space and how accurate are the ICD codes that are hand entered. Another question is whether busy clinicians, who do not use the data on the encounter form for subsequent patient care, completely code all diagnoses made during a patient visit. One study found that, during patient visits, physicians addressed an average 3.05 patient problems but documented only 1.97 on billing forms (they documented nearly as many problems in the paper record as they addressed) (Beasley et al., 2004).
ICD codes, because of their inaccuracy and the fact that their primary use and purpose is billing, are likely to be less than ideal when used for other purposes. One study found low accuracy of billing data about cardiac diseases relative to a clinical research database (Jollis et al., 1993). Another study found that one-third of patients who received an ICD code that indicated the presence of a notifiable disease did not truly have the notifiable disease (Campos-Outcalt, 1990). A third study found that data about prescriptions identified patients with tuberculosis more accurately than all 60 ICD codes for tuberculosis combined (Yokoe et al., 1999).
ICD codes might be less ideal for biosurveillance than other coding systems such as SNOMED-CT. The designers of ICD did not design it with biosurveillance requirements in mind. One study found that SNOMED-CT was superior to ICD for coding ED chief complaints (McClay and Campbell, 2002). SNOMED-CT had a term that was a precise match for 93% of chief complaints; ICD had a precise match for only 40% of chief complaints.
In summary, billing ICD codes from insurance claims and hospital discharge data sets are widely available, but at long time latencies (weeks to months). ICD codes at shorter time latencies ICD (within 24 hours of ED or office visit) are less available. Who assigns ICD codes and when and how influence the time latency, diagnostic precision, and accuracy of ICD-coded data. Thus, it is essential that studies describe the process that generated the ICD codes and measure time latency.
7 Hence the need for professionals to do the coding in the first place.
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