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Mortality Measurement: Development of the 3M™ All Patient Refined Diag

Presentations from a November 2008 meeting to discuss issues related to mortality measures.

By John Hughes

Expanding the Scope of the DRG System

The original objective of the DRGs was to develop a patient classification system that related the types of patients treated to the resources they consumed. Thus, the DRGs focused exclusively on resource intensity. The Centers for Medicare & Medicaid Services (CMS) DRGs (formerly the Health Care Financing Administration [HCFA] DRGs) and the AP-DRGs have remained focused on this limited objective. As the health care industry has evolved there has been increased demand for a patient classification system that can be used for applications beyond resource use, cost, and payment. In particular, a patient classification system is needed for:

  • The comparison of hospitals across a wide range of resource and outcome measures. Such comparisons are typically disseminated to the public by State data commissions.
  • The evaluation of differences in inpatient mortality rates.
  • The implementation and support of critical pathways.
  • The identification of continuous quality improvement projects.
  • The basis of internal management and planning systems.
  • The management of capitated payment arrangements.

In order to meet these needs, the objective of the DRG system needed to be expanded in scope to address patient severity of illness and risk of mortality as well as resource intensity. As previously defined, these patient attributes have the following meaning:

  • Severity of illness. The extent of physiologic decompensation or organ system loss of function.
  • Risk of mortality. The likelihood of dying.
  • Resource intensity. The relative volume and types of diagnostic, therapeutic, and bed services used in the management of a particular disease.

The APR DRGs expand the basic DRG structure by adding four subclasses to each DRG. The addition of the four subclasses addresses patient differences relating to severity of illness and risk of mortality. Severity of illness and risk of mortality relate to distinct patient attributes. For example, a patient with acute choledocholithiasis (acute gallstone attack) as the highest secondary diagnosis may be considered a major severity of illness but only a minor risk of mortality. The severity of illness is major since there is significant organ system dysfunction associated with acute choledocholithiasis. However, it is unlikely that the acute episode alone will result in patient mortality and thus, the risk of mortality for this patient is minor. If additional, more serious diagnoses are also present, patient severity of illness and risk of mortality may increase. For example, if peritonitis is present along with the acute choledocholithiasis, the patient may be considered an extreme severity of illness and a major risk of mortality. Since severity of illness and risk of mortality are distinct patient attributes, separate subclasses are assigned to a patient for severity of illness and risk of mortality. Thus, in the APR DRG system a patient is assigned three distinct descriptors:

  • The base APR DRG (e.g., APR DRG 194 Heart Failure or APR DRG 440 Kidney Transplant)
  • The severity of illness subclass
  • The risk of mortality subclass

The four severity of illness subclasses and the four risk of mortality subclasses are numbered sequentially from 1 to 4 indicating respectively, minor, moderate, major, or extreme severity of illness or risk of mortality. For applications such as evaluating resource use or establishing patient care guidelines, the APR DRG in conjunction with severity of illness subclass is used. For evaluating patient mortality the APR DRG in conjunction with the risk of mortality subclass is used.

Although the subclasses are numbered 1-4, the numeric values represent categories and not scores. For example, severity subclass 4 congestive heart failure patients are not comparable to severity subclass 4 patients with a fractured leg. Thus, it is not meaningful to average the numeric values (i.e., 1-4) of the severity of illness or risk of mortality subclasses across a group of patients to compute an average severity score. However, the APR DRG severity and risk of mortality subclasses can be used to compute an expected value for a measure of interest (e.g., length of stay, cost, mortality), using statistical techniques such as indirect rate standardization.

The underlying clinical principle of APR DRGs is that the severity of illness or risk of mortality subclass of a patient is highly dependent on the patient's underlying problem and that patients with high severity of illness or risk of mortality are usually characterized by multiple serious diseases or illnesses. In the APR DRGs, the assessment of the severity of illness or risk of mortality of a patient is specific to the base APR DRG to which a patient is assigned. In other words, the determination of the severity of illness and risk of mortality is disease-specific. Thus, the significance attributed to complicating or comorbid conditions is dependent on the underlying problem. For example, certain types of infections are considered a more significant problem in a patient who is immunosuppressed than in a patient with a fractured arm. In APR DRGs, high severity of illness or risk of mortality are primarily determined by the interaction of multiple diseases. Patients with multiple comorbid conditions involving multiple organ systems represent difficult-to-treat patients who tend to have poor outcomes.

The Development Process

The process used in the development of the APR DRGs involved an iterative process of formulating clinical hypotheses and then testing the hypotheses with historical data. Separate clinical models were developed for each of the base APR DRGs. Once the clinical model for severity of illness and risk of mortality was developed for each base APR DRG, it was evaluated with historical data in order to review the clinical hypotheses. If there was a discrepancy between clinical expectations and the data results, the clinical content of the ICD-9-CM diagnosis and procedure codes was closely examined to determine if ambiguities in the definition or content of the codes could explain the discrepancy. Any discrepancies between clinical expectations and data results were always resolved by using clinical expectations as the basis for the APR DRGs. Thus, the APR DRGs are a clinical model that has been extensively tested with historical data. The historical data used in the development of version 20.0 of the APR DRGs was a nationwide database of 8.5 million discharges, which included all payer discharges from 1,000 general hospitals from 10 states, and all payer discharges from 47 children's hospitals in the United States. For version 24.0, testing of new diagnosis and procedure codes was conducted using Healthcare Cost and Utilization Project (HCUP) 2003 data which contained over seven million discharges.

Development of the Base APR DRG

The AP-DRGs (see chapter 1) were initially used as the base DRGs in the formation of the initial APR DRGs. A series of consolidations, additions, and modifications were then made to these initial APR DRGs to create the base APR DRGs. Similar to the Yale research, the first step in forming the APR DRGs was to consolidate all age, CC and major CC splits. The APR DRGs also consolidated all splits based on discharge status of death. This was necessary so that death as an outcome variable could be examined across all the APR DRGs.

In addition to these uniform consolidations, the APR DRG system introduced an extensive set of consolidations, additions, and refinements to the initial APR DRG categories. This includes the diagnoses and procedures and birthweight ranges (for newborns) that define an APR DRG, the procedure codes that are considered OR procedures, and the placement of surgical APR DRGs in their respective MDC surgical hierarchies. The APR DRG system has also introduced numerous changes to the definition of MDCs and the pre-MDC hierarchies and categories. Finally, the APR DRG system has introduced a new kind of logic referred to as "rerouting logic," that reassigns a patient to a new MDC and APR DRG in certain circumstances where the principal diagnosis is overly broad or the sequencing of principal and secondary diagnosis is unclear. Altogether these changes result in a set of base APR DRGs that are very different from those of other DRG classification systems. Following is a summary description of these changes.

Consolidate APR DRGs based on complicated principal diagnosis

APR DRGs that were defined based on complicated principal diagnoses were consolidated. For example, in the initial version of APR DRGs, appendectomies with a complicated principal diagnosis (e.g., appendicitis with peritonitis) were assigned to a different APR DRG than uncomplicated appendectomies. The APR DRGs for appendectomies were consolidated and recognition of the complicated principal diagnosis was subsequently incorporated into the subclass assigned within the APR DRG. Other examples of this kind of consolidation include vaginal delivery with complicating diagnoses and other antepartum diagnoses with complicating diagnoses.

Consolidate APR DRGs based upon complicated OR procedures

The APR DRG system consolidated certain surgical categories that, in both the CMS DRGs and AP-DRGs, are subdivided based upon more complicated types of OR procedures. Examples of surgical category consolidations are cholecystectomy with common duct exploration versus cholecystectomy without common duct exploration, and total mastectomy versus subtotal mastectomy. Surgical procedures were consolidated when the different procedures represented fundamentally the same type of patient and the difference in complexity could be captured through the APR DRG severity of illness and risk of mortality subclasses.

Consolidate APR DRGs based on case volume

The general trend toward outpatient surgery made some of the initial APR DRGs extremely low in volume. Such APR DRGs were consolidated into other similar APR DRGs. For example, carpal tunnel releases are now rarely performed on an inpatient basis. Thus, the APR DRG for carpal tunnel release was consolidated into the APR DRG for nervous system procedures for peripheral nerve disorders, which includes procedures such as tarsal tunnel release, and, subsequently, all of these procedures were consolidated into the APR DRG for other nervous system and related OR procedures. Since the early 1990's when the APR DRGs were first developed, there have been many areas where hospitalization rates have decreased. This is examined carefully and in each subsequent update of the APR DRG classification system, there have been a number of further consolidations for low volume APR DRG categories for both medical and surgical patients.

Pediatric additions

While the AP-DRGs incorporated some of the pediatric modifications from the PM-DRGs (chapter 1), the APR DRGs incorporated the remaining significant pediatric modifications in the PM-DRGs. In addition, in conjunction with National Association of Children's Hospitals and Related Institutions (NACHRI), the APR DRGs were reviewed with a national pediatric database. As a result of this review, additional APR DRGs were created. For example, scoliosis (curvature of the back) is one of the primary reasons spinal fusions are performed on pediatric patients. Spinal fusions for scoliosis tend to be more complex than spinal fusions for other clinical reasons such as a herniated disk. Thus, the APR DRG for spinal fusions was subdivided based on a principal diagnosis of scoliosis. Another example is the creation of an APR DRG for major cardiothoracic repair of heart anomaly.

Restructure newborn APR DRGs

The base APR DRGs for newborns were completely restructured. Age was used instead of principal diagnosis to define the newborn major diagnostic category (MDC); birthweight ranges were used as the starting point framework for newborn APR DRGs; surgical APR DRGs were created within each birthweight range; and medical hierarchies were created within birthweight ranges that have more than one medical APR DRG. A medical hierarchy is necessary because newborns do not have a principal diagnosis in the usual sense. Most newborns have a live newborn status code as their principal diagnosis. This does not permit assignment to a medical APR DRG based on principal diagnosis. Thus, it was necessary to create a medical hierarchy for newborns.

As in the AP-DRGs, the APR DRG newborn MDC was initially defined to include all neonates, with age 0-28 days at time of admission. For version 20.0 APR DRGs, the age definition for MDC 15 was redefined and narrowed to be more consistent with its title, "Newborns & Other Neonates with Conditions Originating in the Perinatal Period." MDC 15 is now defined to include patients age 0-7 days and a subset of patients age 8-14 days who are low birthweight patients and may still have perinatal complications during this time period necessitating transfer to another hospital. This removes from MDC 15 virtually all readmissions to the hospital for community acquired infections and other problems that occur after the first week of life. The new age definition for MDC 15 increases the clinical similarity of MDC 15 patients, better aligns MDC 15 patients with the organization of patient care units and physician specialties, allows for the elimination of certain low volume APR DRGs in MDC 15, and places the older neonatal patients (8-28 days) in other MDCs where they can be assigned to more disease specific APR DRGs.

Initially, the newborn MDC was organized into six birthweight ranges—the same as in AP-DRGs. For version 20.0 APR DRGs, the number of birthweight ranges was expanded to eight and the number of different APR DRG categories within each birthweight range was decreased. The net effect of all APR DRG category changes in MDC 15 was a reduction in the number of base APR DRGs from 35 in version 15.0 to 28 in version 20.0.

Version 20.0 of APR DRGs also incorporated the use of gestational age codes that were introduced into ICD-9-CM in October 2002. Gestational age is used as part of the severity of illness and risk of mortality subclass assignment for newborns.

Add APR DRGs for mortality

The same base APR DRGs are used in conjunction with both the severity of illness subclasses and risk of mortality subclasses. Thus, some new APR DRGs were necessary in order to reflect differences in mortality. For example, initial APR DRG 45 (Specific Cerebrovascular Disorders Except TIA) was subdivided into APR DRG 45 (CVA With Infarct) and APR DRG 44 (Intracranial Hemorrhage) as a result of the significantly higher mortality rate for intracranial hemorrhage patients. In version 20.0 APR DRGs, neonates <500 grams (1.1 pounds) were placed in a new APR DRG separate from neonates 500-749 grams (1.1-1.6 pounds) because the mortality rates are so much higher for neonates <500 grams. Other APR DRG additions and refinements

Chapter 1 of the APR DRG Definitions Manual explains that the process of defining the medical and surgical categories in an MDC requires that each category be based on some organizing principle. The end goal is to create categories that are clinically coherent and have sufficient case volume to be useful. Following are examples of ways in which version 20.0 APR DRG modifications improve clinical coherence:

  • Consolidate APR DRGs if there aren't meaningful clinical differences; e.g., combine APR DRG 202 Angina Pectoris and APR DRG 198 Coronary Atherosclerosis.
  • Improve the clinical distinction between related APR DRGs; e.g., redefine APR DRGs 301 and 302 for joint replacement to be based on the joint replaced (i.e., hip versus knee) instead of the etiology (i.e., trauma versus non trauma).
  • For MDC 22 (Burns), re-conceptualize the APR DRGs to give further emphasis to third degree burns.
  • For MDC 24 (Human Immunodeficiency Virus Infections), refine the list of major HIV related conditions and significant HIV related conditions.
  • For MDC 25 (Multiple Significant Trauma), redefine the APR DRGs giving more emphasis to the surgical categories.
  • Throughout the MDCs, consistently define APR DRGs for which the reason for the hospitalization is a complication of treatment. These APR DRGs now exist in MDCs 5, 6, 8, 11, 18, and 21.
  • Throughout the MDCs, refine and make more consistent the definition of Other Related OR Procedures APR DRGs.
  • Substantially redefine the three APR DRGs for OR Procedures Unrelated to Principal Diagnosis so that each is defined by a distinct level of surgical complexity.

Reclassification of OR Procedures

The APR DRG system has reevaluated the procedure codes considered OR procedures which in turn affects whether a patient will be assigned to a surgical or medical APR DRG.

Version 20.0 APR DRGs removed 62 procedure codes from the APR DRG list of OR procedures, leading to two-and-a-half percent fewer patients classified into surgical APR DRGs. The highest impact reclassified procedure is excisional debridement. Next most common is endoscopic lung biopsy followed by certain other biopsies of bone, soft tissue, blood vessel, cervix, uterus, and bladder. Other reclassified procedures with volume are interruption of vena cava and linear repair eyelid laceration. The APR DRGs most affected by these procedure code reclassifications are the APR DRGs previously defined on the basis of skin graft or excisional wound debridement in MDCs 8, 9, 10, and 21 and the "other OR procedure" APR DRGs throughout the various MDCs.

Revise MDC definitions

The APR DRG system has introduced numerous changes to MDC definitions, especially with version 20.0 APR DRGs.

  • The age definition for MDC 15 (Newborns & Other Neonates with Conditions Originating in the Perinatal Period) was narrowed as described previously.
  • MDC 25 (Multiple Significant Trauma) was updated with respect to the lists of significant trauma diagnoses and the introduction of OR procedures to clarify whether certain diagnoses represent significant trauma. The net effect was to decrease the number of MDC 25 medical patients and increase the number of MDC 25 surgical patients.
  • MDC 24 (Human Immunodeficiency Virus Infections) was updated with respect to the definition of HIV related diagnoses, leading to somewhat fewer patients assigned to MDC 24.
  • MDC 21 was redefined and had its title changed from "Injuries, Poisonings & Toxic Effects of Drugs" to "Poisonings, Toxic Effects, Other Injuries and Other Complications of Treatment." The title change reflects that most of the injury diagnoses previously in MDC 21 have been moved to other body system specific MDCs, namely MDCs 1, 3, 5, 8, and 9. "Other Complications of Treatment" was added into the title of MDC 21 since these diagnoses have always been in MDC 21.
  • Cranial and face bone diagnoses, previously dispersed across MDCs 3, 8, and 21, were consolidated into MDC 3 which is reflected in the revised title for MDC 3, "Ear, Nose, Mouth, Throat and Craniofacial Diseases and Disorders."
  • Prematurity diagnoses (for older neonates and infants) and orthopedic aftercare diagnoses were moved to MDC 23 (Rehabilitation, Aftercare, Other Factors Influencing Health Status & Other Health Service Contacts).

In addition, other individual diagnoses were assigned to different MDCs.

Revise MDC surgical hierarchies

The APR DRG system has introduced a number of changes to the MDC surgical hierarchies. Version 20.0 introduced changes to the surgical hierarchies for MDCs 1, 3, 5, 6, and 8. To illustrate, in MDC 6 (Diseases & Disorders of the Digestive System), APR DRG 224 (Peritoneal Adhesiolysis) was moved lower in the surgical hierarchy following the APR DRGs for appendectomy, anal procedures, and hernia procedures because the peritoneal adhesiolysis is usually adjunct to these procedures and not the patient's primary surgical procedure. Most of the patients who remain in APR DRG 224 are having peritoneal adhesiolysis performed for intestinal obstruction.

A similar example in MDC 8 (Diseases & Disorders of the Musculoskeletal System and Connective Tissue), is APR DRG 312 Skin Graft, Except Hand for Musculoskeletal and Connective Tissue Diagnoses, which was moved lower in the surgical hierarchy. It now follows the APR DRGs for knee/lower leg procedures, foot & toe procedures, and shoulder, upper arm & forearm procedures because the skin graft is usually an adjunct to these procedures and not the patient's primary surgical procedure. The skin graft procedure is indicative of the complexity of the procedure and is taken into consideration in the severity of illness and risk of mortality logic that deals with select combinations of OR procedures.

Revise Pre-MDC hierarchies and categories

The initial APR DRGs started with the same pre-MDC hierarchies and categories as AP-DRGs: MDC 15 (Newborns & Other Neonates with Conditions Originating in the Perinatal Period), MDC 24 (Human Immunodeficiency Virus Infections), Transplants, two Tracheostomy APR DRGs and MDC 25 (Multiple Significant Trauma). For version 20.0 APR DRGs, this was reordered as follows: Transplants, MDC 15, Tracheostomy APR DRGs, MDC 25, and MDC 24. The reordering of the pre-MDC hierarchies provided a clearer focus for classifying the most defining aspects of the hospitalization for these patients.

Version 20.0 APR DRGs redefined and narrowed the definition of the two pre-MDC Tracheostomy APR DRGs. The previous approach included virtually all tracheostomy patients with separate APR DRGs based on whether the principal diagnosis pertained to the face, mouth, or neck, implying that the tracheostomy was a therapeutic treatment for an upper airway problem versus all other principal diagnoses, which implies that the tracheostomy was performed to allow the patient to be on extended mechanical ventilation. The new approach requires that all patients assigned to the tracheostomy APR DRGs receive mechanical ventilation 96+ hours and subdivides the tracheostomy APR DRGs based on whether there is an extensive OR procedure. The new approach in effect narrows the definition to patients on extended mechanical ventilation and classifies other tracheostomy patients to the regular APR DRG categories—particularly in MDC 3 (Ear, Nose, Mouth, Throat & Craniofacial Diseases and Disorders).

Rerouting logic

The basic organizing approach to classification in the APR DRG system is to first assign a patient to a Major Diagnostic Group (MDC) based upon principal diagnosis, and then to a specific APR DRG category based upon principal diagnosis (if medical) or operating room procedure (if surgical). This works well in the vast majority of cases and results in the patient being assigned to the MDC and APR DRG that best describes the reason for the hospitalization.

There are several different kinds of situations, however, where using the principal diagnosis as the starting point for establishing the MDC and APR DRG needs to be supplemented by additional information to yield the most useful classification of the patient. One such situation occurs when there is a clear patient characteristic that should take priority, such as for a patient with an organ transplant or a tracheostomy in the absence of an ENT problem. This situation is handled by Pre-MDC assignment logic mentioned above. Another situation occurs when the principal diagnosis is overly broad, or the sequencing of principal diagnosis and secondary diagnosis is unclear, or a surgical procedure provides clarification of the principal diagnosis. These situations are handled through what is referred to as APR DRG "rerouting logic" which considers secondary diagnoses, procedures, and sometimes age, most often in conjunction with the principal diagnosis, to clarify the reason for the hospitalization. The rerouting logic either reassigns the patient to a new APR DRG within the same MDC (Within MDC Rerouting) or to a new MDC and APR DRG (Across MDC Rerouting).

These situations are not unique to the APR DRG classification system. They represent ambiguities that confront any DRG classification system. What is unique to the APR DRG classification system is the rerouting logic developed to assign these patients to the most appropriate and useful category.

An example of a medical rerouting within an MDC is a patient with a principal diagnosis of chest pain and a secondary diagnosis of angina pectoris or coronary atherosclerosis. The chest pain diagnosis is a symptom of the angina or coronary atherosclerosis and should have been recorded as a secondary diagnosis. The rerouting logic will assign this patient to APR DRG 198 Angina Pectoris & Coronary Atherosclerosis instead of APR DRG 203 Chest Pain, and will resequence the diagnosis of angina or coronary atherosclerosis as the principal diagnosis so that these diagnoses do not make a redundant contribution to the severity of illness and risk of mortality subclass assignment.

An example of a medical patient rerouting across MDCs is a patient with a principal diagnosis of hypovolemia (dehydration) and a secondary diagnosis of gastroenteritis. There is some ambiguity in the sequencing of principal and secondary diagnosis, while the patient fundamentally is a gastroenteritis patient who has some level of dehydration. So, in this example there would be a rerouting from MDC 10, APR DRG 422 Hypovolemia to MDC 6, APR DRG 249 Non-Bacterial Gastroenteritis, Nausea & Vomiting.

An example of a surgical patient rerouting across MDCs is amputation. In previous versions of APR DRGs and other DRG systems, there are distinct amputation DRGs in MDCs 5, 8, and 10. Starting with version 20.0, most of these patients are rerouted to MDC 8 (Diseases & Disorders of the Musculoskeletal System and Connective Tissue) and grouped according to the MDC 8 surgical hierarchy. The end result is that clinically similar amputation patients are grouped together rather than dispersed into separate lower volume amputation groups.

The sequencing of principal diagnosis and secondary diagnosis on the patient discharge records is not altered by any of these resequencing processes. Rather, the APR DRG grouper is redesignating principal diagnosis and secondary diagnosis for specified steps that are part of its logic. In the example of principal diagnosis hypovolemia and secondary diagnosis gastroenteritis, the APR DRG grouper resequences principal diagnosis and secondary diagnosis for grouping purposes but when users examine their own discharge records hypovolemia will still be the principal diagnosis. This also means that when users examine their patients in MDC 6 (Diseases & Disorders of the Digestive System) and especially APR DRG 249 Non-Bacterial Gastroenteritis, Nausea & Vomiting, some of the patients will have a principal diagnosis of hypovolemia, which is ordinarily assigned to MDC 10 (Endocrine, Nutritional & Metabolic Diseases and Disorders). A fuller explanation of the APR DRG rerouting logic and a more extensive set of illustrations is in chapter 3.

The end result of the consolidation and refinement process for version 12.0 of the APR DRG classification system released in 1995 was the creation of 382 base APR DRGs (plus two ungroupable or invalid APR DRGs). This was further consolidated to 355 base APR DRGs for version 15.0 released in 1998 and to 314 base APR DRGs (plus two ungroupable or invalid APR DRGs) for version 20.0 released in 2003. For version 25.0, the base APR DRGs remain at 314. The modifications to the base APR DRGs were sufficiently extensive that a complete renumbering of the base APR DRGs was included as part of the version 15.0 update.

There were many changes to the APR DRG category definitions introduced as part of version 20.0 of the APR DRG system. Overall, this reduced the number of base APR DRGs by 41 from 357 to 316 as a result of the elimination of 55 base APR DRGs and the addition of 14 new base APR DRGs. In addition, 66 base APR DRGs had major definitional changes and 102 base APR DRGs had moderate definitional changes. Version 20.0 reduced the number of final APR DRG severity of illness and risk of mortality subclass categories from 1,422 to 1,258 (including two ungroupable or invalid APR DRGs that do not have subclasses).

Once the definition of the base APR DRGs was completed, four severity of illness subclasses and four risk of mortality subclasses for each of the APR DRGs were evaluated and updated for each new release of the APR DRGs.

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Page last reviewed March 2009
Internet Citation: Mortality Measurement: Development of the 3M™ All Patient Refined Diag. March 2009. Agency for Healthcare Research and Quality, Rockville, MD.


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