Renal function tests in a nutshell.

RENAL FUNCTION TESTS

Functions of kidney

  • Excretion of Metabolic Waste Products, Foreign Chemicals, Drugs, and Hormone Metabolites
  • Regulation of water – electrolyte balances, body fluid osmolality
  • Regulation of arterial pressure
  • Regulation of acid-base balance
  • Regulation of Erythrocyte Production
  • Regulation of 1,25–Dihydroxyvitamin D3 Production
  • Gluconeogenesis

OBJECTIVES OF RENAL FUNCTION TESTS

  • To detect possible renal damage and assessment of its severity
  • To diagnose renal disease
  • To observe the progress of renal disease.
  • To assess baseline measurements prior to starting treatment with certain drugs.
  • The type of measurement of kidney function performed will be determined by the reason for assessing renal activity.

Gives information about:

  • Renal blood flow
  • Glomerular filtration
  • Renal tubular function
  • Urinary out flow unhindered by any obstruction

Four groups of tests

  • Urine analysis
  • Concentration & dilution tests
  • Blood chemistry
  • Renal clearance tests

The usual blood test which checks that the kidneys are working properly measures the level of urea, creatinine, and certain dissolved salts.

Urea is synthesised  in  the   liver  as  end  product of the deamination  of  the  amino  acids; its  elimination  in the urine represent  the major route  for  nitrogen  excretion .   It is  filtered from  blood at  the  glomular  .      there fore,    a measurement  of B. urea  &  S. creatinine   give  a  good  evidence  for  the  renal function.   Urea   production  is  increased  by  a  high  protein  in- take   &  by  the    absorption   of  amino   acid   &   peptides  after  gasterointestinal    haemorrhage.     On  the  other hand ,  B .urea decreased  in  low  protein  in take   &  in  liver diseases.

Creatinine is a waste product made by the muscles. Creatinine passes into the bloodstream, and is usually passed out in urine. A high blood level of creatinine indicates that the kidneys may not be working properly. Creatinine is usually a more accurate marker of kidney function than urea.

Estimated glomerular filtration rate (eGFR) provides a guide to kidney function. Although the level of creatinine in the blood is a useful guide to kidney function, the eGFR is a more accurate measure. Blood creatinine can be used to estimate the eGFR using age, sex, and race. This is often calculated by computer and reported with the creatinine blood test. The normal value for eGFR is 90-120 ml/min. An eGFR below 60 ml/min suggests that some kidney damage has occurred. The value becomes lower with increasing severity of kidney damage.

Dissolved salts that are routinely measured are sodium, potassium, chloride and bicarbonate. They are sometimes referred to as ‘electrolytes’. Abnormal blood levels of any of these may be due to a kidney problem. (Some other conditions may also alter the salt balance in the blood.)

URINE ANALYSIS

Under physical, chemical, Bacteriological and Microscopic examination

PHYSICAL EXAMINATION
Assess 24 hour urinery out put [volume], Appearance, Colour, Turbidity, pH, Specific gravity, osmolality etc

Volume

Is a measure of glomerular filtration and tubular reabsorption
Normal urine volume varies from 500 – 2,500 ml
L/24 hr – typical in health
Temperate climates: output of 800-2500 ml urine per day is usual.
Dependent upon subject’s activity, hydration status, diet and body size.
Sudden changes in volume of urine can indicate problems with ability to concentrate urine
Children: ca 1.5 ml/Kg of b.w./1 hour!

Oliguria

Urine volume less than 400 ml/24 hours or < 1 ml/kg In hypotension or hypovolaemia & intrinsic renal pathology Polyuria – urine output of > 2 litres/24 hours

Disturbance in the tubular concentrating capacity or ADH failure [ diabetes insipidus]
Increased osmotic load (diabetes mellitus)
Excessive water intake (physiological response)
Drug-induced (outdated tetracycline, lithium)
Deficiency of vasopressin
Associated with nocturia

Anuria < 100 mL
Total anuria is usually due to obstruction in the urinary tract.

Colour –appearance

Normally Amber light coloured
Very clear urine with high frequency of urination indicates it’s less likely to be a bacterial problem
Deep yellow -Concentrated urine, Jaundice
Red urine- Haematuria, Haemoglobinuria Myoglobinuria, Porphyria, Beet root ingestion, Drugs like rifampicin, pyridium
Cloudy -Infection
Milky – Chyluria Pyuria Phosphaturia
Dark on standing –Porphyria, Alkaptonuria
Turbidity-seen in Infection, Nephrotic syndrome, proteinuria

Osmolality

Measures urine concentrating ability
Normal – 400-900 mOsm/Kg H20
Can reach Max – 1200 mOsm/Kg H20
Useful for determining whether ionic imbalances exist in subject
Depends on # of particles, not size or charge
Largely due to ADH
Prior to collection, fluid intake restricted
First void submitted for evaluation
Measuring using the fact of freezing point depression
Increased -dehydration, DM, hyperglycemia, hypernatremia
Decreased -overhydration, hyponatremia, Diabetes insipidus

Urinary pH

Normally acidic
Normal range – 4.5-8
Diagnostic significance- when it is studied serially in response to acid load in suspected renal tubular acidosis
Iimportant when studying metabolism of various nutrients e.g. glucose during exercise

Specific gravity

A measure of density of urine measured with density of water
With a refractometer or urinometer
Gives rough estimate of osmolarity
Normal -1.003 – 1.030 ; Average – 1.018
The higher the number = the more concentrated urine
A fixed specific gravity of 1.010 is characteristic of chronic renal insufficiency
Iincreased -Lack of fluids, Increased ADH, Glomerular disorders
Falsely high-when Glucose, dye or protein in urine

Decreased -Dilute urine, Decreased ADH [diabetes insipidus ], primary tubular disorders

CHEMICAL TESTS
To assess the permeability of glomerular membrane

To see the presence of Protein,Glucose, RBCs, Hemoglobin, Ketone bodies, Nitrites, bilirubin

Using Dipstick tests -paper strips impregnated with appropriate reagents & indicator dyes
Modern dipsticks with multiplied zones -For Protein, hemoglobin, glucose, urobilinogen, nitrite, leukocytes, specific gravity, and pH etc

Tests for Protein

Proteinuria describes a condition in which urine contains an abnormal amount of protein. Proteins are the building blocks for all our body parts, including muscles, bones, hair, and nails. Proteins that circulate in our bloodstream also perform a number of important functions. They protect us from infection, help our blood clot, and help keep the right amount of fluid circulating around our bodies, so it is important to maintain the correct levels of proteins in our bodies. Our kidneys help do this, when they are healthy.

A 24-hour urine collection and measurement of protein is the most accurate
Normally small amount of protein is excreted in the urine which may not exceed 150 mg/24 hours
Screening tests Dipstix test and acid precipitation test

Who should have their urine routinely tested for proteinuria?

It is suggested that the following people should be offered a urine test for proteinuria:

  • people with kidney function known to be less than 60% of normal
  • people with diabetes
  • people with high blood pressure (hypertension)
  • people with heart and blood vessel (cardiovascular) disease (ischaemic heart disease, chronic heart failure, peripheral vascular disease and cerebral vascular disease)
  • people with complex diseases which may involve the kidneys – for example, systemic lupus erythematosus (this is a disease where a person’s immune system attacks and injures the body’s own organs and tissues)
  • people with a family history of kidney failure or a family history of inherited kidney disease
  • people found to have blood in their urine.

Dipstix test

A paper strip impregnated with bromophenol blue dye which changes to blue in the presence of protein at a suitable pH (pH 3)
As the strip has a yellow background the colour change is observed as green
The intensity of green is proportional to concentration of protein in urine
Disadvantages – colour change is pH-dependent and a highly alkaline urine can induce it
The test has to be done on fresh urine
It has a low sensitivity for other proteins such as globulins and Bence-Jones protein.
The lower limit of detection ranges from 50-100 mg/dl.

Acid precipitation test

A more sensitive but less specific test
Eight drops of sulphosalicylic acid are added to 2 ml of urine
A precipitate forms in the presence of protein
Light chains and low-molecular-weight proteins are detected by this technique.
False positive results occur with penicillin, PAS, etc

Mild proteinuria – chronic interstitial disease, febrile illness and congestive cardiac failure

Small amount – severe urinary tract infection or obvious haematuria

Large amount (3 g/day or more) – glomerular disease.

Urine protein/urine creatinine ratio

When 24 hours’ collection of urine is difficult or impractical as in children or patients with urinary fistulae, urine protein/urine creatinine ratio can be calculated in spot urine sample.
Due to diurnal variation the best sample is obtained at mid morning
A value of < 0.3 is considered normal, 0.3 – 3.0 is abnormal, and > 3 indicates massive proteinuria

Microalbuminuria

Conventional methods cannot detect urinary albumin excretion of 20 to 200 mcg/min, referred to as microalbuminuria
It is a particularly useful test for detecting incipient diabetic nephropathy
The urine sample is collected under standard conditions after rest of 2 hours, overnight (8 hours) or early morning.
A very specific and rapid method is the radio-immunoassay technique
UAE can however increase with exercise, hypertension, cardiac failure, urinary tract infection and after drinking large amounts of fluid
Bence-Jones proteins -are light chains excreted by patients suffering from monoclonal gammopathies. It is not detected by dipstix and is best identified by immunoelectrophoresis of urine

Glucose

Using Benedict’s test
Dipstix are specific
Normally –ve
Positive urine glucose- Increased blood glucose,Low renal threshold,Other tubular diseases
False +ve- Ascorbic acid
Renal glycosuria is not infrequent in the elderly where renal threshold for glucose is lowered below normal of 180 mg/dl or in inherited tubular defects (e.g. Fanconi’s syndrome)

Blood

Very sensitive; 2 or more cells can produce result
Sometimes TOO sensitive, giving false positives
Can’t distinguish between blood and free Hb, so usually double-check with microscope
Nitrites are positive when UTI with gram +ve bacteria

Bacteriological examination

By proper & aseptic collections of mid stream specimen of urine
The presence of any bacteria in suprapubic aspirate should be considered indicative of bacterial infection.Urine cultures should follow
Bacterial counts of more than 105/ml indicate significant bacteriuria
Llower counts cannot be ignored in suprapubic specimen, patients on treatment with antibiotics
Immunosuppressed individuals and symptomatic infection

Urine Microscopy

The genitalia should be cleaned with soap and water and a mid-stream specimen should be asked for.
If a ‘clean catch’ specimen cannot be obtained, it is better to do a suprapubic aspiration
Microscopic examination should be done immediately as delay facilitates bacterial growth and disintegration of cellular components of sediments
Both uncentrifuged and centrifuged samples should be examined
Should check for- Red cells, pus cells, epithelial cells, crystals, urinery casts

WBC cells -0-1 HPF

Presence of more than 5 wbcs/hpf suggest infection, pyelonephritis, inflammation of GUT
Should be complemented with urine cultures

RBC cells [0-1 hpf]

Large number of RBCs with renal diseases, lower urinary tract disease, exercise
Dysmorphic appearance of RBCs in glomerular aetiology

Epithelial cells [0-2 HPF] : Increased in bladder inflammation, tubular injury etc

Casts

Cylindrical bodies formed by coagulation of Tomm-Horsfall glycoprotein within the tubules
Hallmark of renal parenchymal disease
The material contained within the tubular lumen at the time of cast formation gets entrapped within the cast
Often seen normally after exercise
Hyaline casts-non specific ;seen in normal urine
Granular casts-in pyelonephritis
Red cell casts –in a/c glomerulonephritis
WBC casts- in Proliferative GMN,Interstitial nephritis
Waxy – Advanced renal failure
Fatty- Nephrotic syndrome,Fabry’s disease
Mixed- Proliferative GMN (SLE,PAN)
Bacterial – Bacterial pyelonephritis
Broad-Progressive renal failure with compensatory hypertrophy of nephrons
Pseudocasts are composed of clumped urates, leucocytes and bacteria

Crystals

Triple phosphate [coffin lid shaped]and calcium oxalate crystals [envelope shaped] may be found even in normal urine and are not significant per se
Other crystals identifiable in abnormal urine are of cystine, urate, sulphonamides, etc.

CONCENTRATION & DILUTION TESTS

Aim-To evaluate functional capacity of renal tubules
Ability of nephron to do so –dependent upon Functional activity of tubular cells in renal medulla & Presence of ADH
Failure to achieve adequate urinery concentration due to Defects in renal medulla [NDI] Lack of ADH [CDI ]
Traditionally concentration is determined by Specific gravity of urine gives rough estimate of osmolarity

WATER DEPRIVATION OR CONCENTRATION TEST

To diagnose tubular disease in early stage
Artificial fluid deprivation for > 14 hrs
No food or water after 6 p.m on the night preceding the test.
Discard any urine voided during the night & the first specimen- voided at 7.00 a.m
A second specimen – at 8 a.m & tested
If the nephron is normal ,water is selectively reabsorbed & excretion of urine of high solute concentration [SG-1.025 or more] with an osmolality exceeds 850 mOsm/kg
If tubular cells are non functional solute concentration remains constant regardless of stress of water deprivation
The test should not be performed on a dehydrated patient

VASOPRESSIN TEST

Depends only on renal tubular function
At 8 pm-five units of vasopressin tannate is injected subcutaneously
All urine samples are collected separately until 9 a.m. the next morning
Satisfactory concentration is shown by at least one sample having a specific gravity above 1.020, or an osmolality above 800 m osm/kg
This test will often detect impaired function when creatinine clearance is normal, as in hypertension or potassium deficiency

DILUTION TEST/WATER EXCESS TESTS

After an overnight fast the patient empties his bladder completely and is given 1000 ml of water to drink
Urine specimens are collected for the next 4 hours, the patient emptying bladder completely on each occasion
Unless there is renal functional impairment, the patient will excrete at least 700 ml of urine in the 4 hours, and at least one specimen will have a specific gravity less than 1.004.
Kidneys which are severely damaged cannot excrete a urine of lower specific gravity than 1.010 or a volume above 400 ml in this time. There is a delayed diuresis
Abnormal results are also found if there is delayed water absorption or adrenal cortical hypofunction
If renal tubules are diseased the concentration of solutes in the urine remain constant irrespective of excess water intake
The test should not be done if there is oedema or renal failure; water intoxication may result

DYE EXCRETION TESTS

Using Phenolsulphonphthalein (phenol red) or Indigo-carmine
Its excretion essentially tests for renal plasma flow and is therefore impaired early in conditions such as heart failure

BLOOD CHEMISTRY
Impairment of renal function leads to elevation of end products of protein metabolism thus increased accumulation of urea, BUN, & creatinine in blood & azotemia results

Urea

End-product of protein metabolism chiefly excreted through the kidney
It is filtered by the glomeruli and variably reabsorbed in the tubules
The normal plasma concentration is 20-40 mg/dl
Blood urea concentration is about 14% less than plasma concentration.
Blood urea does not rise until a reduction of 50-60% of GFR
The real urea concentration is BUN x 2.14
Raised -High-protein diet , Hypercatabolic states, Surgery, Infection ,Trauma , Steroid therapy , Tetracyclines , Hypotension, dehydration
Low – Low-protein diet , Old age (reduced catabolism]

BUN [blood urea nitrogen]

Normal BUN range is 8-25 mg/dL
It is not possible to detect renal damage by a raised BUN until renal function has fallen by about 50 percent as measured by the creatinine clearance test
Estimation is most useful for the assessment of the severity and progress of renal failure in Acute tubular necrosis, Acute glomerulonephritis, Chronic renal disease, Post-renal obstruction
Decreased BUN -in Low protein diet,Liver damage,Dialysis

Creatinine

The breakdown product of creatine phosphate released from skeletal muscle at a steady rate. It is filtered by the glomerulus.
It is generally a more sensitive and specific test for renal function than the BUN.
Normal range is 0.6-1.3mg/dL
High levels of creatinine associated with high levels of beta 2 microblobulin in the serum as well as urine
Increased-Impaired renal function,Very high protein diet, Anabolic steroid users, Vary large muscle mass: body builders, giants, acromegaly ,Rhabdomyolysis/crush injury.Athletes taking oral creatine drugs

Uric Acid

Metabolite of purine metabolism
Filtered by the glomeruli and both reabsorbed and secreted by the renal tubules.
Normal value-2.4-7.0 mg/dl
Increased -Renal failure,Gout,Liver and sweetbread gourmets,Lead poisoning,Thiazide diuretics
High dose aspirin, Burns, Crush injuries, Severe hemolytic anemia, Myeloproliferative disorders
Plasma cell myeloma, Tumor lysis: post chemotherapy

BUN/creatinine ratio

Of > 15 is abnormal and indicates pre or post renal azotaemia
It is also elevated in all conditions associated with urea overproduction.
A low ratio is found in pregnancy, overhydration, severe liver disease, and malnutrition.

RENAL CLEARANCE TESTS
To assess GFR & renal blood flow

Renal clearance of a substance is the volume of plasma that is completely cleared of the substance by the kidneys per unit time

Clearance principle
If the plasma passing through the kidneys contains 1 milligram of a substance in each milliliter and if 1 milligram of this substance is also excreted into the urine each minute, then 1 ml/min of the plasma is “cleared” of the substance.

Thus, clearance refers to the volume of plasma that would be necessary to supply the amount of substance excreted in the urine per unit time.

Stated mathematically , Cs × Ps = Us × V

Re arranged as Cs = (Us × V)/Ps

Cs = clearance rate of a substance ‘s’
Ps = plasma concentration of the substance
Us = urine concentration of that substance
V = urine flow rate [volume in ml/min]

Thus, renal clearance of a substance is calculated from the urinary excretion rate (Us × V) of that substance divided by its plasma concentration

Estimation of GFR
If a substance is freely filtered and is not reabsorbed or secreted by the renal tubules, then the rate at which that substance is excreted in the urine (Us ×V) is equal to the filtration rate of the substance by the kidneys (GFR × Ps)

Thus, GFR × Ps = Us × V
The GFR, therefore, can be calculated as the clearance of the substance as follows:
GFR = Us × V /Ps = Cs ; Thus here equal to clearance of that substance
Normal GFR -120 + 25ml/min/1.73m2

a. Inulin or mannitol clearrance test

Inulin

Ideal substance
Polysaccharide molecule
Molecular weight of about 5200
Not produced in the body, is found in the roots of certain plants
IIts filtered from the glomerulus & is excreted unchanged in urine
IV infusion of inulin/mannitol is given to maintain constant plasma concentration
Timed urine samples are collected

Creatinine clearance test
Normally released in to plasma by muscle metabolism and is cleared from the body fluids almost entirely by glomerular filtration.

Because measurement of creatinine clearance does not require intravenous infusion into the patient, this method is much more widely used than inulin clearance for estimating GFR clinically

Creatinine clearance is not a perfect marker of GFR because a small amount of it is secreted by the tubules so that the amount of creatinine excreted slightly exceeds the amount filtered

Easy method & routinely employed method of GFR estimation

Method
A careful and accurate 24 hour collection of urine is made at some time during the day (but not within 1-3 hours after a large meal)

A blood sample is taken for plasma creatinine analysis

Creatinine clearance = ( Ucr × volume [ml] x 1.73/A )  / Pcr × time [min]

Ucr = Urine creatinine concentration
Pcr = Plasma creatinine concentration
V = Urine flow in ml/min
A = Body surface area in m2 and
1.73 is the standard body surface area

Body surface area can be measured with the help of height and weight charts or with the help of following formula. log A = 0.425 log Wt + 0.725 log Ht – 2.144

According to Cock Croft and Gaut formula – When Errors in 24 hours urine collection, creatininine clearance is calculated from plasma creatininine concentration which incorporates age, sex, and weight to estimate Ccr from plasma creatinine levels without any urinary measurements

Creatinine clearance = 140 – age (years) X wt (kg) ) x 72 for men

Serum creatinine (mg/dl)

For women, the estimated GFR is multiplied by 0.85 because muscle mass is less.

This formula overestimates GFR in patients who are obese or edematous, and is most accurate when normalized for body surface area of 1.73 m2.
Normally creatininie clearance is 90-130 ml/min in an adult of normal size
Approximately 100 mL/min/1.73 m2 in healthy young women and 120 mL/min/1.73 m2 in healthy young men.
The Ccr declines by an average of 0.8 mL/min/yr after age 40 years as part of the aging process, but 35% of subjects in one study had no decline in renal function over 10 years.
Values below 90 ml/min are indicative of diminished glomerular filtration rate seen with renal insufficiency
Increased values in pregnancy,exercise etc
The test has particular value in the general assessment of renal function in cases when plasma analyses are invalid, such as after dialysis, or when the BUN has been lowered by a low protein diet
Creatininie clearance & GFR are inversely proportional to plasma creatinine concentration
If GFR suddenly decreases by 50%, the kidneys will transiently filter and excrete only half as much creatinine, causing accumulation of creatinine in the body fluids and raising plasma concentration
If GFR falls to one-fourth normal, plasma creatinine would increase to about 4 times normal, and a decrease of GFR to one-eighth normal would raise plasma creatinine to 8 times normal

Urea clearance test

No need of IV infusion
Less sensitivity test Cz plasma concentration of urea is affected by number of factors
Like dietary protein,fluid intake, inflammation, trauma, surgery, corticosteroids
Partly reabsorbed from tubules
Other substances for GFR estimation- radioactive iothalamate and EDTA and DTPA (=both derivates of acetic acid) & cystatin C

Para amino hippuric acid [PAH] clearance tests

To measure renal blood flow
When IV infusion of PAH, both filtration at glomerulus & secretion by tubules
If a substance is completely cleared from the plasma, the clearance rate of that substance is equal to the total renal plasma flow
Because the GFR is only about 20 per cent of the total plasma flow, a substance that is completely cleared from the plasma must be excreted by tubular secretion as well as glomerular filtration
The percentage of PAH removed from the blood is known as the extraction ratio of PAH and averages about 90 per cent in normal kidneys
Therefore, the clearance of PAH can be used as an approximation of renal plasma flow
In diseased kidneys, this extraction ratio may be reduced because of inability of damaged tubules to secrete PAH into the tubular fluid
Clearance of PAH = 585 ml/min
Total renal plasma flow = Clearance of PAH/Extraction ratio of PAH
Thus Total renal plasma flow is 650 ml/min
If the hematocrit is 0.45 and total renal plasma flow is 650 ml/min
Renal blood flow is calculated, RBF = 650/(1 – 0.45) = 1182 ml/min

Other measures-

Renal failure indices

Two important diagnostic indices are the renal failure index and fractional excretion of filtered sodium.
Renal failure index (RFI) = Urine sodium (mEq/L) x Plasma creatinine (mg/dl]
Urine creatinine (mg/dl)
Fractional excretion of filtered sodium (FENa) calculated as = Urine sodium (mEq/L) x plasma creatinine (mg/dl) x 100
Plasma sodium (mEq/L) x urine creatinine (mg/dl)
In prerenal failure, RFI and FENa are < 1

To see the structural integrity of renal system
Plain X-ray KUB, IVP , Cystoscopy ,Excretion urography ,Ultrasonography,CT,MRI, Antegrade pyelography , Retrograde pyelography ,Micturating cystourethrography (MCU), Aortography or renal arteriography ,Renal scintigraphy – dynamic and static, Transcutaneous renal biopsy etc can be done.

Summary

  • Examination of the urine is the most important initial test for suspected renal damage, particularly glomerular diseases
  • Search must be made for protein, erythrocytes and casts.
  • The urine concentration test (or vasopressin test) is sensitive. It is possibly the most useful single test for confirming the presence of renal tubular impairment
  • The creatinine clearance is quantitative for glomerular impairment and needs rarely be done unless simpler tests are abnormal.
  • The estimation of plasma urea or creatinine should be done as a guide to progress and prognosis if there is severe renal damage or obstruction

3 Comments

  1. Dr. Shafeeq says:

    Great Article. Thank you!

  2. mintu says:

    Excellent…Please help us with an easy to grasp ABG analysis next sir….I haven’t seen anyone explain ABG as easy as Bijoy sir can

    • Dr.Bijoy says:

      I will soon post.

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