Secondary Diagnosis
Secondary diagnoses are the additional confirmed conditions, reported in OASIS item M1023 and on the claim, that coexist with the primary diagnosis and affect the patient's care. Under PDGM they determine the comorbidity adjustment, which can raise the 30-day period's case-mix weight when qualifying conditions or interacting condition pairs are present.
How secondary diagnoses affect payment
PDGM includes a comorbidity adjustment with three levels: none, low, and high. A low adjustment applies when the claim carries a secondary diagnosis from a qualifying comorbidity subgroup; a high adjustment applies when two or more secondary diagnoses fall into subgroup pairs that interact to predict higher resource use. The subgroup lists and interactions are recalibrated in rulemaking, most recently using CY2024 data for CY2026. The OASIS captures up to 5 secondary diagnoses in M1023, but the claim can carry substantially more, and the claim is what CMS uses for the adjustment, so thorough claim coding matters even when OASIS slots run out.
What qualifies as a reportable secondary diagnosis
Report conditions that are active and relevant: they require treatment or monitoring, affect the patient's responsiveness to treatment, or influence the plan of care. Resolved conditions and incidental history do not belong. Every reported code needs practitioner confirmation somewhere in the record, since home health agencies cannot establish diagnoses independently. The discipline is completeness without padding: undercoding leaves legitimate comorbidity adjustments and clinical context on the table, while stacking unsupported codes to chase a high comorbidity adjustment is an audit and False Claims Act exposure.
Secondary diagnoses beyond payment
Comorbidities do more than adjust payment. They feed risk adjustment models that set expected values for outcome measures, so incomplete coding can make your patients look healthier than they are and your outcomes look worse than they deserve. They also shape the plan of care: a heart failure patient whose diabetes and chronic kidney disease go uncoded tends to have those conditions undermanaged in visit focus, teaching, and early-warning monitoring. Referral sources and case managers read the coded record too, and it should reflect the real complexity your clinicians are managing.
Building a reliable secondary coding process
Agencies that code comorbidities well share a few habits:
- Reconcile hospital discharge summaries, medication lists, and referral records against coded diagnoses
- Query the practitioner when medications or findings imply an unconfirmed condition
- Sequence codes so actively managed conditions appear on the claim
- Keep OASIS diagnoses, the plan of care, and the claim consistent
- Audit a sample of episodes for both missed comorbidities and unsupported codes
Frequently asked questions
How many secondary diagnoses can be reported?
OASIS item M1023 captures up to 5 secondary diagnoses, and the claim can carry more, up to the claim format's limit of 24 secondary positions. The comorbidity adjustment is calculated from the claim, so conditions beyond the OASIS slots still count when properly coded and supported.
What is the difference between low and high comorbidity adjustment?
Low applies when at least one secondary diagnosis falls in a qualifying comorbidity subgroup. High applies when secondary diagnoses form qualifying interacting pairs associated with higher resource use. High outweighs low, and only one adjustment level applies per 30-day period.
Do secondary diagnoses have to be treated by the home health agency?
They must be relevant to the patient's care: actively managed, monitored, or affecting the plan of care or treatment response. A condition can qualify without being the focus of visits, but purely historical, resolved conditions should not be coded.