RENAL FUNCTION TESTS

By doctoroid

1) Excretory – primary :by urine formation

2) Regulation of volume & electrolyte composition of ECF

3) Regulation of acid-base balance 4) Endocrine function – produce &

secrete: erythropoietin, renin, calcitriol(1,25-DHCC)

5) Site of neoglucogenesis – not primary: in starvations- esp. from glutamine

collective term for a variety of individual tests and procedures that can be done to evaluate how well the kidneys are functioning.

Primarily reflects two basic mechs.– Glomerular ultrafiltration & Tubular reabsorption/secretion

Practically, divided into 3 groups –1) Analysis of urine & blood2) Specific assessment of renal clearance3) Additional special Tests

Early detection of possible renal damage & assessment of its severity

Measure progression of the renal impairment & efficacy of corrective therapy

Predict when renal replacement therapy may be necessary

Monitor safe & effective use of drugs, which are principally eliminated through urine.

A) PHYSICAL :1)Volume > 800-2500 ml/dintake~2.5

L/d Polyuria >2.5L Chronic GN Anuria ,Oliguria2) Appearance > clear Turbid (alkalinity d/t prolonged standing

l/t ppt of Ca/Mg-phosphates,↑phosphate , presence of pus d/t UTI)

3) Colour> straw/amber-yellow urochrome

Brownish yellow (jaundice) Dark (alkaptonuria) Reddish brown (RBC/Hb/Mb-uria,Porphyria

etc.)

4) Odour> mild aromatic volatile org. acids Unpleasant ammoniacal (prolonged standing) Acidotic fruity (DKA)

5) Sp. Gravivity & Osmolality > 1.003 to 1.030 & 50-1200 mOsm/kg

(depends on state of hydration of the body) Early morning urine sample(=after

overnight fast)if SG>1.018 & Osm>600 ≡Normal

SG is simplest to measure but unreliable(in presence of HMW substances) for evaluating renal concentrating ability.

SG decreased,increased & fixed(1.010=CRF)

1) Reaction > mild acidic pH avg.6 (=4.5-7.5)

normal short PP alkaline tide Protein rich diet acidic Vegetable rich diet alkaline also in

type II DTA, UTI by urease producing organisms, Acetazolamide therapy, alkali ingestion.

2) For abnormal urinary constituents :

I) Proteins > Normal upto 150 mg/d—routinely

undetected Proteinuria albumin predominates By– a) heat & acetic acid test b) Sulphosalicylic acid test c) Esbach’s albuminometer

II) Reducing Sugars > Normally absent –

glucose/fructose/galactose When renal threshold is exceeded By Benedict’s Test

III) Blood > Normally does not appear By Benzidine Test

IV) Ketone Bodies > Normally not present By- Rothera’s Test & Gerhardt’s test.

V) Bile salts > Only in early phases of obstructive

jaundice By- Hay’s test & Petenkoffer’s test

VI) Urobilinogen > N ~1 - 3.5 mg/d ↑ in persistent fevers, hepatobiliary

diseases, haemolytic jaundice By- Ehrlich’s test & Schlesinger’s test

VII) Bile-pigments > Bilirubinuria=↑conj.Bilirubin hep/post-

hep jaun By- Modified Fouchet’s Test

Imp findings in the urinary sediment includes---

I)Casts >> proteinaceous plugs

Formation favoured by sluggish flow Various shapes c/t tubules in which

formed cellular or non-cellular Types Hyaline, RBC, WBC,

Granular, Broad waxy etc.

II) Crystals >> Ca-oxalate/phosphate, Triple phosphate--

common May be normally found risk of stone in

future Urate or Cysteine crystals pathologic

III) Cells >> RBCs, WBCs, pus cells, Sq.epithelial,

Tubular epithelial cells

Strip impregnated with reagents for the substances in question within a urine sample.

By comparing the colour-change(in the paper-squares)with the standardized colour-charts.

Modern dipsticks with multiplied zones: Can detect/measure: Protein, hemoglobin,

glucose, urobilinogen, ketones, leukocytes, specific gravity, and pH

A promising tool everywhere at the level of primary care!!!

There is no plasma constituent whose conc. depends solely on the functionality of kidneys.

Frequently used are 2 normal metabolic wastes Excreted by kidneys accumulates in renal

dysfunction ↑blood levels

I) Blood Urea Nitrogen >> 8-25 mg% begin to rise only after 50% renal damage

II) Plasma Creatinine >> 0.6 – 1.5 mg% More reliable as BUN is subjected to variations

Vol. of plasma that is cleared of a substance in unit time, by its’ urinary excretion ml/min

Calculated as: C = UV/P Predominantly determine GFR: Relationship

as—

Correlated more directly with the status of kidney function employed to assess GFR,RPF & RBF

GFR = C

No reabs, No Secret

INULIN

GFR > C Much reabs, No Secret

Gluc, AA, Na+, Cl-

GFR < C No reabs, Much Secret

PAH, Diodrast

Characteristics of an Ideal Marker : Constant rate of production (or for exogenous

marker can be delivered IV at a constant rate) Freely filterable at the glomerulus (minimal

protein binding) No tubular reabsorption/secretion No extrarenal elimination or metabolism Availability of an accurate & reliable assay For exogenous markers-- safe, convenient,

readilyavailable, inexpensive & physiologically inert

Various markers used :A) Exogenous >>1) Inulin (gold standard but technically

demanding)2)Non-radiolabelled contrast media (e.g.

Iohexol) 3)Radiolabelled compounds (e.g. 99m Tc-DTPA)

B) Endogenous >>1)Creatinine (marginally overestimates—most

widely used in clinical practice)2)Urea (one of the 1st markers– not used at

present)

Approximation of bedside GFR with limited accuracy by “Cockroft & Gault formula”

Most widely used & best validated for adultsCcr =(140-Age)x(Wt in Kg)/(Plasma

Creatinine x72) [Correction factor for females = 0.85] value to such formulas for GFR prediction is

likely to increase when an accurate plasma creatinine assay is performed along with inhibition of tubular secretion by cimetidine/probenecid.

Applying “Fick’s Principle” to kidney :

Amount of a sub excreted by kidney in unit time(UV) =RPF X renal A-V diff. in its plasma conc.(Pa - Pv)

RPF(ml/min) =UV / (Pa - Pv)

Criteria of the marker to be used : Almost totally extracted from plasma with each

passage through kidney Not metabolised/stored/produced by kidney Physiologically inert & easily assayable

Use of PAH Clearance to measure RPF/RBF:

Cont. low dose PAH inf. plasma conc. Constant All PAH excreted in

urinePv(PAH)=0eliminated ≡> RPF = UV/Pa(PAH) = Clearance of PAH(C-PAH) 10% RPF perfuses non-excretory portionsERPF True RPF = ERPF/0.9 RBF = true RPF / (1 – Haematocrit value)

Normal ERPF = 600-650 ml/min/1.73 sq.mt BSA Approx. RBF = 1200 ml/min

A) TESTS FOR TUBULAR FUNCTIONS:I) Urine Conc. Test >> Early dinner no food/fluid after 6 PMbladder

emptied @ 7AM discarded specimens collected @ 8 AM & 9AMatleast one should hv SG >1.022 or Osm >850 mOsm/kg

II) Vasopressin test >>No fluid after 6 PM s.c.

ADH(5U)inj.@8PMurine samples collected separately till 9AMatleast one should SG>1.020 or Osm>800

III) Urine Dilution Test >>Pt. completely empties bladder after overnight

fast drinks 1L waterhourly urine specimens collected for next 4 hrsatleast 700ml will be excreted & atleast one should hv SG <1.004

IV) Urine Acidification Test >>Fasting from midnightcomplete bladder

emptying @morningOral Am.Cl.(0.1gm/kg) with 1L water given hourly urine samples collected for next 6 hrs. atleast one should hv pH of 5.3 or less

V) Dye Excretion Test or PSP Test>>

Phenolsulphonphthalein(Phenol red)— filtetred & secreted.

600 ml water drink f/b IV 6mg PSPhourly urine samples collected40-60% should be excreted in 1st hr. & another 20-25% should excrete in 2nd hr

Excretion<50% over 2hrs. abnormal Useful for detecting early stage of renal dis.

VI) Other Sophisticated Methods>>

MICROPUNCTURE techniquesMICROCRYOSCOPIC studiesMICROELECTRODE studies

VII) Renal Biopsy >>Specimen subjected to LM,EM & IFM-studies↑knowledge & better understanding of

renal diseases

Plain radiograph of abdomen IVPUSG, CT Scan, MRI ScanRadionuclide studies

Strictly speaking, these are not considered to be RFTs, but very useful in present day clinical practice for structural & functional assessment of kidneys.