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| Exogenous substances | Characteristics and advantage | Disadvantages |
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| Inulin (MW 5200 Dalton) | (i) Considered the gold standard for the estimation of GFR.
(ii) It is freely filtered by glomerulus and is neither reabsorbed nor secreted by the renal tubules.
(iii) Metabolically inert and cleared only by the kidney.
(iv) It requires constant IV infusion to maintain plasma level and once steady state has been achieved, plasma and timed urine specimen levels are measured. | Analysis of inulin is technically demanding, time-consuming, labor intensive, costly, and unsuitable for outpatient use. |
| Nonradiolabelled contrast media (iothalamate/iohexol) | It has the advantage that urography and an estimation of GFR can be done at a single examination. | Cannot be used for routine to assessment of GFR. |
| Radiolabelled compounds | Radiolabelled compounds include [51Cr] EDTA, [125I] iothalamate, [99Tcm] DTPA, and [131I] Hippuran. | Cannot be used for routine to assessment of GFR. |
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| Endogenous substances | | |
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| Urea (MW 60 dalton) | One of the first markers for assessing GFR | Urea production is variable and is influenced by many substances. |
| Creatinine (MW 113 daltons) | First introduced by Popper and Mandal [27] | (i) Dependent on muscle mass, which varies with age, gender, and weight.
(ii) Some secretion by the renal tubules.
(iii) Difficulty of obtaining complete, accurately timed urine collections.
(iv) High protein intake leads to 10% increase in plasma creatinine.
(v) Drugs like triamterene, spironolactone, amiloride, probenecid, cimetidine, trimethoprim, high dose salicylates, or pyrimethamine inhibit tubular secretion and induce true elevation of plasma creatinine. |
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