Physiology is the key to better diagnosis coding.
By Nancy Reading, RN, BS, CPC, CPC-P, CPC-I
The renal system consists of two kidneys (each of which usually has an adrenal gland perched on top), two ureters, a bladder, and a urethra. This article focuses on renal failure and the physiology behind code selection.
Renal disease usually results from damage to one of four major structures of the kidney: the tubules, the glomeruli, the interstitium, or the intrarenal blood vessels. Renal disease is classified as acute or chronic.
Today, the term acute kidney injury (AKI) replaces the term acute renal failure (ARF). AKI equates to an abrupt decline in renal function, associated with a 50-80 percent mortality rate.
The etiologies for AKI are often described as pre-, intra-, or post-renal.
Pre-renal problems occur before the kidney, and usually cause a drop in perfusion of the kidneys without compromising the integrity of the parenchyma. Causes of intra-renal AKI are usually due to diseases of the renal vessels, diseases of the renal microcirculation and glomeruli, effects of ischemia or nephrotoxic drugs, and/or tubo-interstitial inflammation. An obstruction further downstream (e.g., in the ureter) can put backward pressure on, and cause damage to, the nephron as a post-renal etiology.
Manifestations of renal failure can be found in simple blood chemistry studies that measure serum creatinine, blood urea nitrogen (BUN), and glomerular filtration rate (GFR). A decrease in GFR with an elevation of serum creatinine and BUN are hallmarks of renal disease. The severity of the problem is based on how far from normal these parameters have changed.
Rating Renal Function Severity
There is a variety of ranking systems for rating acute renal failure. Three systems that classify acute kidney injury (AKI) are:
- Risk, Injury, Failure, Loss of Kidney Function, and End-stage Kidney Disease (RIFLE);
- Acute Kidney Injury Network (AKIN); and
- Kidney Disease Improving Global Outcomes (KDIGO).
There is only one diagnosis code for AKI, regardless of how severe it is. AHA Coding Clinic (4th quarter 2008, pages 192-193) tells us to use ICD-9-CM 584.9 (Acute kidney failure, unspecified) for non-traumatic acute kidney injury. The index in ICD-9-CM supports coding AKI to 584.9.
Note that different physicians may have different interpretations of the meaning of acute renal insufficiency versus acute renal failure (ARF). It’s generally accepted that renal insufficiency (593.9 Unspecified disorder of the kidney and ureter) refers to the early stages of renal impairment, determined by mildly abnormal elevated values of serum creatinine or BUN, or diminished creatinine clearance. Clinical symptoms or other abnormal laboratory parameters may be present, but are usually minimal.
If the physician is not specifically documenting ARF or AKI, do not code 584.9; however, if clinical indicators are present for ARF or AKI and only acute renal insufficiency is documented, a query is needed.
Chronic kidney disease (CKD) is a mixed bag of conditions characterized by changes in kidney structure and function. The manifestation of these conditions is based on the underlying cause and severity of the disease.
According to the National Kidney Foundation, Kidney Disease Outcomes Quality Initiative (NKF KDOQI) for renal diseases, the list of clinical parameters shown in the table below is provided for staging CKD. The provider must document the stage and be queried in the absence of documentation.
Stages of Chronic Kidney Disease
|Stage||Description||GFR (mL/min/1.73 m2)|
|1||Kidney damage with normal or ↑ GFR||≥ 90|
|2||Kidney damage with mild ↓ GFR||60 – 89|
|3||Moderate ↓ GFR||30 – 59|
|4||Severe ↓ GFR||15 – 29|
|5||Kidney failure||< 15 (or dialysis)|
|Chronic kidney disease is defined as either kidney damage or < 60 mL/min/1.73 m2 for ≥ 3 months. Kidney damage is defined as pathologic abnormalities or markers of damage, including abnormalities in blood or urine tests or imaging studies.|
Coding the Stages of CKD
The appropriate code for stages 1-5 is assigned to depict the documented severity of CKD:
- Stage 1 codes to N18.1 Chronic kidney disease, stage 1 (ICD-9-CM, 585.1 Chronic kidney disease, Stage I).
- Stage 2 codes to N18.2 Chronic kidney disease, stage 2 (mild) (ICD-9-CM, 585.2 Chronic kidney disease, Stage II (mild)).
- Stage 3 codes to N18.3 Chronic kidney disease, stage 3 (moderate), (ICD-9-CM 585.3 Chronic kidney disease, Stage III (moderate)).
- Stage 4 codes to N18.4 Chronic kidney disease, stage 4 (severe), (ICD-9-CM, 585.4 Chronic kidney disease, Stage IV (severe)).
- Stage 5 codes to N18.5 Chronic kidney disease, stage 5, (ICD-9-CM, 585.5 Chronic kidney disease, Stage V ).
Code N18.6 End stage renal disease is assigned only when the provider has documented end-stage renal disease (ESRD). Encounters
where both a stage of CKD and ESRD are documented, report
N18.6, only. The same holds true in ICD-9-CM when assigning 585.6 End stage renal disease, stage V requiring chronic dialysis when both CKD and ESRD are addressed in a single encounter.
Kidney Transplants with CKD
Patients who have had a renal transplant can still suffer some form of CKD. This does not equate to a transplant complication, according to AHA Coding Clinic. The coder would assign transplant status V42.0 Kidney replaced by transplant and the documented level of CKD for ICD-9-CM.
When attaching an ICD-10-CM code, assign the appropriate N18 Chronic kidney disease (CKD) code for the patient’s stage of CKD and Z94.0 Kidney transplant status.
Coders are directed to only use the complication of transplant code when the complication affects the function of the transplanted organ, followed by ICD-9-CM code 996.81 Complications of transplanted kidney or ICD-10-CM code T86.10-T86.19 (complications of kidney transplant), as appropriate. For either of the complication code set, ICD-9-CM or ICD-10-CM, use an additional code for the transplant complication, such as graft versus host disease (D89.81_ or 279.5_).
Factors Leading to CKD
CKD is often due to nephrotic syndrome. Nephrotic syndrome is associated with overexcretion of protein in the urine (proteinuria); edema of lower extremities, face, and abdomen; and damage to the blood vessels of the nephron. Only assign the code for nephrotic syndrome when the physician specifically states the patient has it. See N04.1-N04.9 in ICD-10-CM or 581.81-581.9 in ICD-9-CM for the appropriate code assignment.
Hypertension is one of the leading causes of CKD. Both ICD-10-CM and ICD-9-CM presume a cause-and-effect relationship between hypertension and CKD. You are directed to combine the two when the chart indicates the patient has both hypertension and CKD. The exception to this rule is when the provider specifically states the two are not related.
ICD-9-CM Official Guidelines for Coding and Reporting guidance for assigning hypertensive CKD codes directs you to assign codes from category 403 Hypertensive chronic kidney disease, when conditions classified to category 585 Chronic kidney disease are present with hypertension. Accurate reporting for a diagnosis of hypertensive CKD (403) requires selection of the appropriate fourth digit to indicate whether the hypertension is classified as malignant (0), benign (1), or unspecified (9). Report the appropriate code from category 585 to identify the stage of CKD.
ICD-10-CM is identical in the requirement to provide two codes for hypertensive renal disease, as well as in the presumption that if they occur together there is a cause-and-effect relationship. There is no longer a distinction between benign, malignant, and unspecified hypertension in ICD-10-CM. Code I12.0 Hypertensive chronic kidney disease with stage 5 chronic kidney disease or end stage renal disease or I12.9 Hypertensive chronic kidney disease with stage 1 through stage 4 chronic kidney disease, or unspecified chronic kidney disease is assigned with the appropriate N18._ code.
The Renal System’s Role
The kidneys maintain a stable metabolic environment in the body. They provide regulation of acid and base balance, excretion of metabolic wastes, and conservation of nutrients. Balance between water and ions such as potassium, sodium, chloride, magnesium, phosphate, and calcium are maintained through conservation and excretion.
The kidneys also play a role in the endocrine system. Secretion of renin regulates blood pressure; secretion of erythropoietin regulates production of erythrocytes; and secretion of 1,25-dihydroxyvitamin D3 regulates calcium metabolism.
Each kidney contains approximately 1.2 million nephrons, the functional unit of the kidney. A nephron is comprised of five distinct units:
2. Proximal convoluted tubule
3. Loop of Henle
4. Distal convoluted tubule
5. Collecting duct
The glomerulus is a cluster of capillaries that loop together to form Bowman’s capsule. The capillaries are held together with Mesangial cells, creating a wall of glomerular capillaries. This wall serves as the glomerular filtration membrane.
The membrane separates the blood in the capillary bed from the fluid filtering out into Bowman’s space (the space inside Bowman’s capsule). The glomerular filtrate passes through three layers of the glomerular membrane to form urine.
The rate of ultrafiltration through the glomerulus depends on many variables. The glomerular filtration rate (GFR) is often used as a measure of renal health. It’s normally greater than (>) 60.
Nancy Reading, RN, BS, CPC, CPC-P, CPC-I, has held a Registered Nurse license for 36 years, earned a Bachelor of Science in Biology/Chemistry, and has 26 years of coding experience. She has worked the gamut of the industry, from a large university practice with over 1,000 providers to Medicaid. Reading is a member of the Salt Lake South Valley, Utah, local chapter.