Functions of the kidney

• Regulation e.g. homeostasis ,water, acid/base

• Excretion e.g. uric acid, urea, creatinine

• Endocrine e.g. renin, erythropoietin, Calcitriol or (1,25-dihydroxyvitamin D3 )- conversion only in kidney!

When should you assess renal function?

• Older age• Family history of Chronic Kidney disease (CKD)• Decreased renal mass• Low birth weight• Diabetes Mellitus (DM)• Hypertension (HTN)• Autoimmune disease• Systemic infections• Urinary tract infections (UTI)• Nephrolithiasis• Obstruction to the lower urinary tract• Drug toxicity

Introduction

• Nitrogen containing compounds that are not proteins or polypeptides

• Total NPN can be tested by making a protein-free filtrate

• Useful clinical information is obtained from individual components of NPN fraction

Uric Acid

• Uric acid is formed from the breakdown of nucleic acids and is an end product of purine metabolism.

• Purines are found in some foods and drinks, such as liver, anchovies, mackerel, dried beans and peas, beer, and wine.

• Purines are also a part of normal body substances, such as DNA.

• Uric acid is transported by the plasma from the liver to the kidney, where it is filtered and where about 70% is excreted.

• The remainder of uric acid is excreted into the GI tract.

• High levels of uric acid in the body is called hyperuricemia

• Normal values fall between 3.0 and 7.0 mg/dL.

• Note: Normal values may vary slightly from laboratory to laboratory.

It is ordered during

• when a doctor suspects high levels of uric acid :

• Gout

• monitoring test when a patient has undergone chemotherapy or radiation

• If a patient appears to have failing kidneys

Greater-than-normal levels of uric acid (hyperuricemia) may be due to:

• Alcoholism

• Diabetes

• Gout

• Hypoparathyroidism

• Lead poisoning

• Leukemia

• Nephrolithiasis

• Renal failure

• Toxemia of pregnancy

• Purine-rich diet

• Excessive exercise

• Chemotherapy-related side effects

Lower-than-normal levels of uric acid may be due to:

• Fanconi syndrome

• Wilson's disease

• Syndrome of inappropriate antidiuretic hormone (SIADH) secretion

• Multiple Sclerosis

• Low purine die

Gout

• Gout is a kind of arthritis that occurs when uric acid builds up in the joints.

• In Gout increased serum levels of uric acid lead to formation of monosodium urate crystals around the joints.

• Acute gout is a painful condition that typically affects one joint.

• Chronic gout is repeated episodes of pain and inflammation, which may involve more than one joint.

Symptoms of acute gouty attacks:

• Symptoms develop suddenly and usually involve only one or a few joints. The big toe, knee, or ankle joints are most often affected.

• The pain frequently starts during the night and is often described as throbbing, crushing, or excruciating.

• The joint appears warm and red. It is usually very tender.

• There may be a fever.

• The attack may go away in several days, but may return from time to time. Additional attacks usually last longer.

• Tophi are lumps below the skin around joints or in other places. They may drain chalky material. Tophi usually develop only after a patient has had the disease for many years.

Clinical Significance

• Measurement of uric acid is used most commonly in the evaluation of renal failure, gout, and leukemia.

• In hospitalized patients, renal failure is the most common cause of elevated uric acid levels, and gout is the least common cause.

• Hypouricemia is seldom observed and associated with rare hereditary metabolic disorders.

Specimen

• Serum or plasma may be used; slight but insignificant positive bias (0.2 mg/dL) has been noted in plasma specimens as compared with serum.

• Stability in serum / plasma:

– 6 months at -20°C

– 7 days at 4-8°C

– 3 days at 20-25°C

Possible Complications

• Chronic gouty arthritis

• Kidney stones

• Deposits in the kidneys, leading to chronic kidney failure

Enzymatic Colorimetric

• Uric acid + H2O + O2 Allantion + CO2 + H2O2

• TBHBA + 4- Aminoantipyrine + 2H2O2 Quinoneimine + 3 H2O

• Uric acid is oxidized to allantoin by uricase. • The generated hydrogen peroxide reacts with 4-

aminophenazone/ESPT to quinoneimine.

POD

Uricase

Treatment

• Treatments for a sudden attack or flare-up of gout:

• Your doctor will recommend that you take nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen, naproxen, or indomethacin as soon as your symptoms begin

• Your health care provider may occasionally prescribe strong painkillers such as codeine

• Daily use of allopurinol decrease uric acid levels in your blood

Some diet and lifestyle changes may help prevent gouty attacks:

• Avoid alcohol, sardines, oils, organ meat (liver, kidney, and sweetbreads(

• Limit how much meat you eat at each meal.

• Avoid fatty foods such as salad dressings, ice cream, and fried foods

• Eat enough carbohydrates

• If you are losing weight, lose it slowly. Quick weight loss may cause uric acid kidney stones to form.

Note:

• Uric acid may also form kidney stones

• Although uric acid can act as an antioxidant, excess serum accumulation is often associated with cardiovascular disease.

Urea

• Highest concentration of NPN in blood (45%)• Major excretory product of protein metabolism• Urea is synthesized in the liver, from CO2 and

ammonia, as the final product of amino acid catabolism.

• It is freely filtered at the glomerulus, though 40% is passively reabsorbed by the proximal tubules.

• The reference interval for serum urea of healthy adults is 5-39 mg/dl (slightly higher in males than females)

• BUN = (blood urea nitrogen)• The real urea concentration is BUN x 2.14

• Blood urea levels can vary proportionately with:

the protein content of the diet,

rate of protein catabolism during tissue breakdown,

and liver function.

• Because of its metabolism, urea is a nonspecific indicator of renal function.

• Blood urea levels are usually elevated before significant changes in creatinine levels have occurred.

• Monitoring blood urea levels is very useful when one is following the course of renal disease.

Clinical Significance

• States associated with elevated levels of urea in blood are referred to as uremia or azotemia.

• Causes of urea plasma elevations:

Prerenal: renal hypoperfusion

Renal: acute tubular necrosis

Postrenal: obstruction of urinary flow

• Parallel determination of urea and creatinine is performed to differentiate between pre-renal and post-renal azotemia.

• Pre-renal azotemia, caused by e.g. dehydration, increased protein catabolism or decreased renal perfusion, leads to increased urea levels, while creatinine values remain within the reference range.

• In post-renal azotemias, caused by the obstruction of the urinary tract, both urea and creatinine levels rise, but creatinine in a smaller extent.

• With obstruction, both plasma urea and creatinine increase, but there is greater rise of urea than of creatinine because the obstruction of urine flow backpressure on the tubule and back diffusion of urea into blood from the tubule.

Increased protein catabolism:• Increased dietary protein• Severe tress: fever, etc• Rhabdomyolysis• Upper GI bleeding

Causes of urea plasma decrease

• Decreased dietary protein• Increased protein synthesis ( Pregnant women ,

children )• severe liver disease• Overhydration (IV fluids)

Specimen• Serum and heparinized plasma can be used for the

urease/GLDH methods.

• Fluoride will inhibit the urease reaction; therefore methods employing urease cannot use serum preserved with fluoride.

• Ammonium heparin also cannot be used as an anticoagulant for urease methods.

• Stability in serum or plasma: 7 days at 4–8°C 1 year at -20°C

• Because of urea’s susceptibility to bacterial degradation, serum and urine samples should be kept at 4° to 8° C until analysis.

Urease/GLDH Method

• The method is optimized for 2-point kinetic measurement.

• Decrease in absorbance at 340 nm is proportional to concentration of urea

Urea / Creatinine Ratio– Pre-renal BUN is more susceptible

to non-renal factors

– Post-renal

Both BUN and Creat. are elevated

Increased BUN

Normal Creat

Increased BUN

Increased Creat

• Creatinine is a non-protein nitrogen waste product formed in muscle.

Creatine Phosphate – phosphoric acid = Creatinine

Creatine – water = Creatinine

• Creatine is synthesized in liver from some a.a (glysine, methionine, arginine)

• Filtered by kidney and excreted in the urine• Creatinine filters easily into the glomerular filtrate and is not

reabsorbed by the tubule. • The plasma levels of creatinine are related to the muscle

mass.

Creatinine

Clinical SignificanceElevated Creatinine is found in

• Impaired renal function

• Very high protein diet

• Vary large muscle mass: body builders, giants, acromegaly patients

• Rhabdomyolysis/crush injury

• Drugs:

• Probenecid

• Cimetidine

• Triamterene

• Trimethoprim

• Amiloride

Clinical Significance

• For renal transplant patients, an increase in serum creatinine of 2 mg/L has been used as a criterion of establishing rejection.

• In other persons a change in creatinine of 2 mg/L would represent a 20% loss in renal function.

Specimen

• One can analyze serum, plasma, or diluted urine.

• The common anticoagulants (fluoride and heparin) do not cause interference, though heparin, which can be formulated as the ammonium salt, must be avoided in enzymatic methods that measure ammonia production.

• Storage

7 days at 4-25oC

At least 3 months at -20oC

Specimen

• Urine should be diluted 1:100

• Bacterial contamination has been found to falsely lower creatinine values measured using the Jaffé reaction.

• The mechanism of this interference appears to be bacterial production of a substance that retards the rate of the Jaffé reaction.

Enzymatic Method

• Creatinine + H2O Creatine

• Creatine +ATP Creatine-P + ADP

• ADP + Phosphoenolpyruvate ATP + Pyruvate

• Pyruvate + NADH Lactate + NAD+

• The difference in absorbance at fixed times during conversion is proportional to the concentration of creatinine in the sample

Lactate dehydrogenase

Creatine Kinase

Pyruvate Kinase

Creatinine aminohydrolase

Creatinine Clearance

• Creatinine clearance is used to estimate the glomerular filtration rate (GFR).

• One method of determining GFR from creatinine is to collect urine (usually for 24-hours) to determine the amount of creatinine that was removed from the blood over a given time interval.

• Clearance is defined as the (hypothetical) quantity of blood or plasma completely cleared of a substance per unit of time.

• The most frequently used clearance test is based on the measurement of creatinine.

• Creatinine is chosen because it is freely filtered at the glomerulus and is not reabsorbed by the tubules.

• However, a small amount of the creatinine (about 5%) in the final urine of healthy persons is derived from tubular secretion.

• To do the test, one needs a precisely timed urine collection and a blood sample taken during the collection period.

• Best results are obtained from a 24-h urine collection.

• The test is initiated by having patients empty their bladder at the beginning of the timed period.

• Urine is collected throughout the period, the bladder is again emptied at the end of the time period.

• The 'clearance' of creatinine from plasma is directly related to the GFR if:

• The urine volume is collected accurately

• There are no ketones or heavy proteinuria present to interfere with the creatinine determination.

• It should be noted that the GFR decline with age (to a greater extent in males than in females) and this must be taken into account when interpreting results.

Creatinine Clearance

• Creatinine determinations are performed on both samples. The creatinine clearance is calculated from the following formula:

• A person has a plasma creatinine concentration of 0.01 mg/ml and in 1 hour produces 60ml of urine with a creatinine concentration of 1.25 mg/mL.

• Creatinine clearance (mL/min)= (UV)/P X 1.73/S

• where U is urinary creatinine (mg/L), V is volume of urine (mL/min), P is plasma creatinine (mg/L), S is the calculated surface area of the patient, and 1.73 is the surface area (m2) of a standard 70 kg person.

• The range of creatinine clearance in healthy persons corrected to a surface area of 1.73 m2 is 90 to 120 mL/min.

• At low filtration rates, the creatinine clearance does not parallel true glomerular filtration rate because a relatively large portion of the urine creatinine is secreted rather than filtered.