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ICD-10-CM is an alphanumeric code, containing seven characters.

Every code starts with a letter followed by two numbers.

The first three characters indicate the category, which describes the general type of disease or injury.

Next, there is a decimal point and the subcategory.

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They are followed by one or two subclassifications that explain the cause, locations, manifestation, severity, and the type of disease or injury.

The last character indicates an extension.

The extension means the type of encounter.

For instance, if it was the first time the healthcare provider saw the patient for this disease, injury, or conditions.

This is listed as the “initial encounter”.

Every next encounter is listed as a “subsequent encounter”.

The visits that relate to the effects of a previous disease or injury are listed as “sequela”.

So, the first digit of this code is always in alpha, and the second is always numeric.

Digits from three to seven can be both alpha and numeric.

For example:

A01 – {Disease}

  • A01.0 {Disease} of the lungs
    • A01.01 … simple
    • A01.02 … complex
      • A01.020 … affecting the trachea
      • A01.021 … affecting the cardiopulmonary system
        • A01.021A … initial encounter
        • A01.021D … subsequent encounter
        • A01.021S … sequela

The ICD-10-CM manual has three volumes.

The first one comes in the form of a tabular index.

The second one is the alphabetic index.

The third includes the codes of procedures used only by hospitals.

The code set is divided by the type of disease or injury into ranges:

  • A00-B99 – Certain infections and parasitic diseases.
  • C00-D49 – Neoplasms.
  • D50-D89 – Diseases of the blood and blood-forming organs and certain disorders involving the immune mechanism.
  • E00-E89 – Endocrine, nutritional and metabolic diseases.
  • F01-F99 – Mental, Behavioral and Neurodevelopmental disorders.
  • G00-G99 – Diseases of the nervous system.
  • H00-H59 – Diseases of the eye and adnexa.
  • H60-H95 – Diseases of the ear and mastoid process.
  • I00-I99 – Diseases of the circulatory system.
  • J00-J99 – Diseases of the respiratory system.
  • K00-K95 – Diseases of the digestive system.
  • L00-L99 – Diseases of the skin and subcutaneous tissue.
  • M00-M99 – Diseases of the musculoskeletal system and connective tissue.
  • N00-N99 – Diseases of the genitourinary system.
  • O00- O9A – Pregnancy, childbirth, and puerperium.
  • P00-P96 – Certain conditions originating in the perinatal period.
  • Q00-Q99 – Congenital malformations, deformations, and chromosomal abnormalities.
  • R00-R99 – Symptoms, signs, and abnormal clinical laboratory findings, not elsewhere classified.
  • S00-T88 – Injury, poisoning, and certain other consequences of external causes.
  • V00-Y99 – External causes of morbidity.
  • Z00-Z99 – Factors influencing health status and contact with health services.

This is an example that shows the level of detail that ICD-10 codes can go to:

Injury: Closed fracture of distal phalanx of the right index finger.

  • S00-T88 – Injury, poisoning and certain other consequences of external causes 
    • S60-S69 – Injuries to the wrist, hand, and fingers
      • S62 – Fracture at wrist and hand level 
      • S62.0 – fracture at the navicular [scaphoid] bone of the wrist
        • S62.5 – fracture of the thumb
        • S62.6 – fracture of other and unspecified finger(s)
        • S62.60 – fracture of unspecified phalanx of finger
        • S62.61 – displaced fracture of proximal phalanx of finger
          • S62.63 – displaced fracture of distal phalanx of finger
          • S62.630 – Displaced fracture of distal phalanx of the right index finger
          •  S62.630A – … initial encounter for closed fracture
          • S62.630B – … initial encounter for open fracture
          • S62.630D – … initial encounter for fracture with routine healing
    • Etc.

So, as you can see, the ICD-10-CM code set is designed for the coders to go to a high level of specificity.

This set also allows you to document laterality (which side the infection or injury is on) and increase the amount of diagnosis information.



Besides the organization and format of the ICD-10-CM, there are also conventions that guide the coders to the right diagnosis codes.

They include:

  • Brackets [ ].
  • Parentheses ( ).
  • “Includes”.
  • “Excludes”.
    • This one has a slight variation because ICD-10-CM has two types of “Excludes” conventions:
      • Excludes1: contains the codes that shouldn’t ever be used with the codes above.
        These are some types of “hard excludes”.
      • Excludes2: contains codes for injuries or conditions that can be a part of a condition but aren’t included here.
        These are kind of “soft excludes”.
        This excludes acting like a “see also” note.
  • “Code first”.
  • “Use Additional Code”.
  • “In Disease Elsewhere Classified”.
  • “See”.
  • “See Also”.
  • “Not Elsewhere Classified”.
  • “Not Otherwise Specified”.

Excludes notes in ICD-10-CM are divided into two levels.

Excludes1 indicates that the codes in the note by no means can be listed with the code that contains the note Excludes1.

It can look like this:

  • A12 {Disease} A.
    • Excludes1.
      • {disease} B, {disease} C.

Excludes1 lists the conditions that are not mutually exclusive with the main conditions that the coder is looking up.

It tells the coder that if the code in the Excludes1, they can’t use the code that houses them under no circumstances.

So if a coder is looking up the {disease} B but considers the code for {disease} A to be appropriate, the Excludes1 directs them to look anywhere else but {disease} A.

Another Excludes note is Excludes2.

This note shows that the code above doesn’t include other conditions found below the note.

Here’s another example:

  • A12 {Disease} D
    • Excludes2
      • {disease} E, {disease} F

The Excludes2 means that while Diseases E and F can be related to Disease D, they aren’t listed in the same code as Disease D.

Conditions listed in Excludes2 can be coded with the condition above the note, unlike Excludes1.

You can consider it as a “see also” note while Excludes1 is like a “see” note.

There is another essential convention in the ICD-10-CM related to code’s extensions.

Extensions usually provide information on which encounter this is for the patient and the healthcare provider.

They don’t have to be included all the time, but when they do, you can’t just append them to the end of the code.

Extensions are found only in the ICD-10-CM code’s seventh character.

If coders need to add an extension for an initial encounter to the code that doesn’t contain six characters, they have to use a placeholder character.

They use an ‘X’ as a placeholder digit.

For instance, if a coder is coding the case of poisoning by unintentional overdose of the antibiotic penicillin, they use T36.0X1A.

Here the fifth digit is empty, so the placeholder ‘X’ is used.

They are used only when an extension is required.

Mostly, ICD-10 codes do not include the encounter extension.

ICD10CM codes

How to Use ICD-10-CM

Coders begin the coding process by analyzing and abstracting the medical report.

Using the notes they took from the report, they can go right to the tabular section or the alphabetic one to find the right code and confirm it with the tabular.

Here’s an example:

The patient is a Caucasian male, 44 years old.

The height and weight reported by the patient are 1.8m and 80 kg.

No notable medical history.

The patient has a red rash around the labial folds and nose.

Some yellowish-reddish pimples.

The patient complains about flaking skin and itching.

The patient reports that the rash appeared two months ago but then subsided.

Diagnosed with seborrheic dermatitis and prescribed a topical antifungal medication.

This is a relatively straightforward visit, and to code it, the coder abstracts the information from the report of the doctor.

The patient has one specific symptom – a rash on the face – and the doctor makes a positive diagnosis: seborrheic dermatitis.

The coder can look it up in the alphabetic index or the tabular index section for skin diseases or subcutaneous tissue: L00-L99.

Then, they look up dermatitis and eczema and get L21: “seborrheic dermatitis.”

Under that category, they will find four subcategories.

Coder will select the one that describes the condition that the physician diagnosed in the best way.

In this case, it would be L21.9, “Seborrheic dermatitis, unspecified.”

Here “unspecified” is used because other codes for seborrheic dermatitis either relate to infants or describe “other” seborrheic dermatitis.

So, “unspecified” is the best option in this case.

Here’s the tree of codes for this diagnosis code:

L00-L99 – Diseases of the skin and subcutaneous tissue

  • L21 – Seborrheic Dermatitis.
    • L21.0 – Seborrhea capitis.
    • L21.1 – Seborrheic infantile dermatitis.
    • L21.8 – Other seborrheic dermatitis.
    • L21.9 – Seborrheic dermatitis, unspecified.

You can see that this code doesn’t have any extensions or subclassifications.

Not all codes need the level of specificity provided by the ICD-10-CM codes.

In this case, we only need the fourth digit to describe the diagnosis.


Further Explorations

Let’s look at another example, this time an injury.

Injuries are usually documented with extensions to specify the encounter between the doctor and the patient.

Whether it’s the initial encounter and no treatment was received, or a subsequent encounter and some treatment were received, it is essential as it can put a great impact on the medical necessity on a claim.

The patient has bruising and swollen nose and cheek after a rugby match.

The patient did not lose consciousness.

Examination shows no fracture of the skin on the face.

X-rays confirm a type II Le Fort fracture.

(A Le Fort fracture is one of the three types of fractures on the facial bones, that include the lower and mid maxillary bones and the cheek/zygomatic arch bone).

Right away, the coder knows it’s an injury code.

So they look up injury codes in S00-T88: “Injury, poisoning and certain other consequences of external causes.”

Then, they move on the S00- S09, “Injuries to the head.”

In that subfield, the coder finds S02, “fracture of the skull and facial bones.”

They can also look it up in the alphabetic section by searching for a Le Fort fracture.

As mentioned, it can be of three types.

From the doctor’s report, the coder knows they are looking for the Type II.

Below S02, the coder finds multiple subcategories, including codes for fractures of the base of the skull and the vault, fractures of the orbital floor, and fractures of the nasal bones.

We need a very specific type, which includes the maxillary and zygoma bones of the face.

There’s a specific subcategory that includes this: S02.4, “fracture of the malar, maxillary and zygoma bones.”

In this subcategory, the coder finds a subclassification for the Le Fort fractures (S02.4) and three more for every Le Fort fracture type.

They pick the code for Type II: S02.412.

Since this is the first encounter between the doctor and the patient, the coder uses the initial encounter extension ‘A’.

So they’d get: S02.412A, “Le Fort type II fracture, closed, initial encounter.”

Here’s how the code tree would look like:

S00-T88 – Injuries, poisonings, and certain other consequences of external causes.

  • S02 – Fracture of skull and facial bones.
  • S02.0 – Fracture of vault of the skull.
  • S02.1 – Fracture of base of the skull.
  • S02.2 – Fracture of nasal bones.
  • S02.4 – Fracture of malar, maxillary and zygoma bones.
  • S02.40 – Fracture of malar, maxillary, and zygoma bones, unspecified.
  • S02.41 – Le Fort fracture.
    • S02.411 Le Fort I fracture.
    • S02.412 Le Fort II fracture.
      • S02.412A – … initial encounter for closed fracture.
      • S02.412B – … initial encounter for an open fracture.
      • S02.412D – … subsequent encounter for fracture with routine healing.
      • Etc.

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