Renal Function Test (RFT)

Original Editor - Manisha Shrestha Top Contributors - Manisha Shrestha, Lucinda hampton and Joao Costa

Original Editor - User Name

Top Contributors - Manisha Shrestha, Lucinda hampton and Joao Costa  

Introduction

The kidneys are responsible for the urinary excretion of uremic toxins and the regulation of several body systems such as intra and extracellular volume status, and acid-base status. They adapt quantitative and qualitative composition of the urine to keep these systems in balance. Along with that kidneys helps in the production of

• erythropoietin which stimulates the production of red blood cell.
• renin for blood pressure regulation
• carries out the conversion of vitamin D to its active form, ultimately helps in calcium absorption.

Renal function test (RFT), also known as kidney function test is used to assess the functions of kidney. It is used screen for, detect, evaluate and monitor acute and chronic kidney diseases. These are simple blood and urine tests that can identify problems with your kidneys.

Need of RFT

RFT is ordered, if your doctor

• thinks your kidneys may not be working properly which is known from signs and symptoms
• and if you have other conditions that can harm the kidneys, such as diabetes or high blood pressure

Signs and symptoms related to kidney problems are:

• Blood in urine (reddish urine)

• Lethargy and weakness
• Dry and itching skin
• Increase urge to urinate (specially at night)
• Foamy urine
• Persistent puffiness of body ( especially around eyes)
• Poor appetite
• Muscles cramping
• Discomfort during urination

Co-morbidities such as diabetes and hypertension usually cause chronic kidney diseases in long term so to plan the treatment, RFT plays a vital role.

Tests of renal function have utility in

• Identifying the presence of renal disease
• Monitoring the response of kidneys to treatment
• Determining the progression of renal disease

Components of RFT

It is divided into two main types based on it's aim:

Aim	To assess the renal diseases	To assess the kidney function
1.	Serum parameters ·         Serum urea ·         Creatinine ·         Electrolytes ( Na+, K+)	Glomerular function ·         Clearance test
2.	Urine analysis	Glomerular permeability ·         Proteins
3.		Tubular function ·         Concentration test ·         Dilution test

Test to assess the kidney function

It is very essential to know the function of kidney and also the process of urine formation to know the rational behind these test and also to interpret the result of these test.

Glomerular Function- Clearance test

The best test to assess glomerular function is Glomerular filtration rate (GFR), which is the rate in milliliters per minute at which substances in plasma are filtered through the glomerulus; in other words, the clearance of a substance from the blood. The normal GFR for an adult male is 90 to 120 mL per minute.

GFR cannot be measured directly so various substances are used to assess GFR. There are various exogenous and endogenous substances that are used for clearance test. The characteristics of an ideal marker to measure GFR are as follows:

• Constant rate of production (or for exogenous marker can be delivered intravenously at a constant rate)
• Freely filterable at the glomerulus (minimal protein binding)
• No tubular reabsorption
• No tubular secretion
• No extrarenal elimination or metabolism
• Availability of an accurate and reliable assay
• For exogenous marker: safe, convenient, readily available, inexpensive, and does not influence GFR (physiologically inert)

Clearance (C) is the rate at which an indicator substance is removed from plasma per unit concentration; specifies a volume from which all of a substance is removed per unit time. For a substance Z cleared by renal elimination:

Cz =UzxV ⁄ Pz

Where Cz is the clearance rate of Z substance, Uz is urinary concentration of z, Pz is plasma concentration of Z, and V is urine flow rate.

There are various exogenous and endogenous substances/markers that are used for clearance test and they are:

Inulin Clearance Test

Since there is no such endogenous marker currently exists, exogenous markers ( most commonly inulin, a polysaccharide- a fructose polymer) is considered the reference method for the estimation of GFR. First intravenous(IV) bolus of inulin is given then continuous infusion of inulin is given so that a constant inulin concentration is maintained in body fluid. Then samples of plasma and urine are taken and the rate of clearance of inulin is determined using clearance rate formula mentioned above.

It is the best marker for clearance test as it is freely filtered through from glomerulus, neither reabsorption nor secreted by renal tubules, and it is neither synthesized nor stored in body so it gives values near to the GFR i.e. about 125ml/minute.

Creatine Clearance test

Creatine phosphate is present in skeletal muscles which is continuously metabolized in to creatine with the wear and tear of muscles. Metabolism of creatinine:

• Generated in muscle by non-enzymatic conversion of creatine and phosphocreatinine
• Generation is proportional to muscle mass and is relatively constant
• Important role of liver in formation of creatinine through methylation of guanidine aminoacetic acid

Levels of creatine varies according to diurnal and menstrual variations, race, body surface area and diet (and method of meat preparation).As GFR increases in pregnancy, lower creatinine values are found in pregnancy.

Factors affecting serum creatine are:

Increase serum creatine	Decrease serum creatine
Ketotic states, Hyperglycemia Cephalosporins Flucytosine (enzymatic method) Cimetidine, trimethoprim (block secretion) Vigorous exercise Ingesting cooked meats ( 30% increase after the ingestion of red meat)	Dietary protein restriction Muscle wasting, malnutrition Bilirubin Renal disease Advanced age Female sex Advanced liver disease

Not an ideal marker since it also is excreted by tubular secretion. Proportion of total creatinine clearance (Ccr) due to tubular secretion increases as GFR decreases and Ccr leads to GFR overestimation by approximately 10-20% . However, it is the most commonly used endogenous marker for the assessment of glomerular function.

Formula to estimate creatine clearance via Cockcroft-Gault equation predicts Ccr as

Ccr=(140-age) (weight)/(72 *SCr) (multiply by 0.85 if female)

Urea Clearance test

Urea or Blood nitrogen test (BUN) is a nitrogen-containing compound formed in the liver as the end product of protein metabolism and the urea cycle. About 85% of urea is eliminated via kidneys; the rest is excreted via the gastrointestinal (GI) tract.

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