How ICD9 classified into 5 Levels?

Question

I was reading this paper

https://arxiv.org/pdf/1802.00382.pdf

and from this paper I quote

Restrict the labels to the most common Level 5 codes, method used by some project reports.7 We start by selecting the 20 most common codes (see Figure 2)

Can someone explain to me what are the 5 levels?

When I look at ICD9 I can see these 4 Levels only

Level 1 :

I   001–139 Infectious and Parasitic Diseases
II  140–239 Neoplasms
III 240–279 Endocrine, Nutritional and Metabolic Diseases, and Immunity Disorders
IV  280–289 Diseases of the Blood and Blood-forming Organs
V   290–319 Mental Disorders
VI  320–389 Diseases of the Nervous System and Sense Organs
VII 390–459 Diseases of the Circulatory System
VIII    460–519 Diseases of the Respiratory System
IX  520–579 Diseases of the Digestive System
X   580–629 Diseases of the Genitourinary System
XI  630–679 Complications of Pregnancy, Childbirth, and the Puerperium
XII 680–709 Diseases of the Skin and Subcutaneous Tissue
XIII    710–739 Diseases of the Musculoskeletal System and Connective Tissue
XIV 740–759 Congenital Anomalies
XV  760–779 Certain Conditions originating in the Perinatal Period
XVI 780–799 Symptoms, Signs and Ill-defined Conditions
XVII    800–999 Injury and Poisoning

Level 2 :

583 Nephritis and nephropathy,
596 Other disorders of bladder

Level 3 :

596.0 Bladder neck obstruction
596.4 Atony of bladder
596.5 Other functional disorders of bladder

Level 4 :

596.51 Bladder hypertonicity
596.52 Bladder compliance, low
596.54 Neurogenic bladder, NOS
596.55 Detrusor sphincter dyssynergia

So where is the 5th level?

Answer 1

A good way to understand the 5 levels that are described in the paper might be to go to a website that presents information on the entire ICD-9 and permits the user to “click though” the levels of codes. Here is a good place to do this.

http://icd9.chrisendres.com/index.php

The hierarchy for the ICD-9 includes first the choice to classify based on Diseases and Injuries or on Procedures. Both are part of the ICD-9 Classification System. This "choice" is (unintuitively) the first level.

Thus, using terminology in the paper, Level 1 is Diseases and Injuries (as opposed to Procedures). Level 2 is the Chapter where each chapter includes a large block of codes all of which mostly share the same first digit and pertain to an organ system (13 chapters) or another way to logically group diseases and conditions. Level 3 is a block of grouped three digit codes where the grouping reflects similar disease processes within the organ system. Level 4 is a specific three digit code for one disease or a set of related diseases or conditions within the Level 3 block. Level 5 is defined by the digits to the right of the decimal place of the Level 4 three digit code and comprises closely related but different diseases or conditions.

In the example given in the cited paper,

Level 1 is: Diseases and Injuries (not Procedures)

Level 2 is: Chapter 17. INJURY AND POISONING (800-999)

Level 3 is: OTHER AND UNSPECIFIED EFFECTS OF EXTERNAL CAUSES (990-995)

Level 4 is: 992 Effects of heat and light

Level 5 is: 992.0-992.8 (multiple codes. For 992, all have 1 digit to right of the decimal place but some three digit--level 4--codes have two digits to the right of the decimal place and some have no digits to the right of the decimal place)