Don't Let "Due To" Boss You Around: How ICD-10-CM Really Handles Etiology vs. Manifestation

Have you ever seen a note like "AKI due to dehydration" and thought, "Okay, so dehydration has to go first because it's the cause — right?"

Not so fast.

In ICD-10-CM, there's a big difference between what the provider writes — clinical cause-and-effect language like "due to," "secondary to" — and what the code set actually requires through the etiology/manifestation convention with "code first" and "use additional code" notes.

Once you see that difference, a lot of confusing sequencing debates suddenly calm down.

Step 1: Separate Two Ideas in Your Brain

Let's name the two concepts so they're easier to keep straight.

1. Clinical relationship — This lives in the documentation. Phrases like "due to," "secondary to," "resulting from," and "associated with" tell you one condition caused another. This is gold for choosing the right code or combination code and for linking conditions.

2. Etiology/manifestation convention (ICD-10-CM rule) — This lives in the book, not the progress note. You'll see it as:

• "Use additional code to identify..." under the etiology
• "Code first underlying condition." under the manifestation
• Manifestation code titles that say "in diseases classified elsewhere"

If you don't see those conventions in ICD-10-CM, you are not in a required etiology/manifestation sequencing situation — no matter how many times the provider writes "due to." This is when you need to determine which condition was primarily responsible for the admission.

Step 2: The "AKI Due to Dehydration" Myth

Let's use your favorite troublemaker: "Acute kidney injury (AKI) due to dehydration."

Here's what many coders are taught — or absorb quietly:

"If it says AKI due to dehydration, then dehydration is the cause, so dehydration has to be principal."

That sounds logical clinically, but ICD-10-CM doesn't say that. In reality:

• There is no special etiology/manifestation pair set up between AKI and dehydration.
• There are no "code first" / "use additional code" instructions that force one before the other.
• So sequencing goes back to the basic inpatient rule: Which condition was chiefly responsible for the admission and the resource use?

If the story is "this patient was admitted for severe volume depletion, got big fluid resuscitation, and AKI resolved pretty fast" — dehydration might very well be the principal diagnosis. If the story is "this patient has severe AKI needing nephrology, close monitoring, possible dialysis" — AKI might be principal.

Either can be principal depending on the circumstances of admission. "Due to" tells you they're linked; it does not force a fixed sequence.

For new coders: Think: "The doctor's 'due to' tells me the relationship; the book tells me if there's a required order."

For seasoned coders: If you've ever seen a local policy that says "we must always make AKI the PDx when it's 'AKI due to dehydration'" — that's exactly the kind of myth this article is pushing back on. There are Coding Clinics that support this sequencing dilemma being situation-dependent.

Step 3: A True Etiology/Manifestation Pair — Malignant Pleural Effusion

Now let's look at a real etiology/manifestation relationship: "Malignant pleural effusion due to lung cancer."

Here's what's different:

• Cancer is the underlying cause (etiology).
• The malignant pleural effusion is considered a manifestation of that cancer.
• ICD-10-CM treats that effusion code as a manifestation-only type code, with instructions that the underlying malignancy must be coded first.

Here, the book and the documentation agree: provider says "malignant pleural effusion due to lung cancer," and ICD-10-CM says "put the lung cancer code first, then code the malignant pleural effusion."

This is true etiology/manifestation: etiology must be sequenced first, manifestation second. There is no "either can be principal" here — if you put malignant pleural effusion as PDx, you're fighting the conventions and facing denials and DRG errors.

Step 4: Quick "Gut Check" Examples

Use these to train your instincts:

• "AKI due to dehydration" — Clinical causality only. No forced etiology/manifestation sequencing. PDx depends on reason for admission and treatment focus.
• "Malignant pleural effusion due to lung cancer" — True etiology/manifestation. Underlying malignancy first, malignant effusion second. Effusion code cannot be principal.
• "Pneumonia due to influenza" — Often captured by a combination code (e.g., "influenza with pneumonia"). "Due to" helps you pick which code, not how to sequence two separate codes.
• "Heart failure due to hypertension" — Hypertensive heart disease codes often bake that relationship into one combination code. Again, "due to" = relationship; ICD-10-CM = combination code and usual PDx rules.
• "Anemia in chronic kidney disease" — This one really does have instructions that the CKD is coded first, and the anemia is added as a linked condition. You're back in required etiology/manifestation territory.

For training, it's powerful to put these examples side-by-side and ask: "Which ones have 'code first / use additional code' patterns in the book, and which are just good documentation with regular sequencing rules?"

Step 5: A Simple Rule-of-Thumb You Can Teach

Give your coders this quick two-question test:

1. Ask: "Do I see 'code first' and 'use additional code' instructions between these two conditions in ICD-10-CM?"
   • If YES — you're in true etiology/manifestation land. Follow the required order.
   • If NO — you're not in a mandatory etiology/manifestation pair, even if the doctor wrote "due to."
2. Then ask: "For inpatient, what truly occasioned the admission?"
   • That's your principal diagnosis.
   • The other clinically significant condition becomes an additional diagnosis.

This instantly reduces the intimidation factor for new coders and gives experienced coders the language to unlearn "always/never" myths.

Step 6: How to Frame This in a Team Huddle

You could put it to your team this way:

"When you see 'due to,' think: Great — now I know these conditions are related. Now I have to open the book and see if ICD-10-CM has special instructions for this pair. If it doesn't, I go back to the principal diagnosis definition and the story of the admission."

That's calmer, more logical, and far less intimidating for someone still learning.

Remember: code book instructional notes and guidelines take precedence. Clinical documentation must always support code choices, but our instructional notes and guidelines must always support the sequencing of those codes. When in doubt, ask for a second opinion. And when in doubt about which condition was more severe and prompted the inpatient admission, query the attending for clarification.