Biological samples were assayed in duplicate using the 96-well protocol. The urea concentration (mg/dL) was 12.5 ± 0.9 for Commercial 2% reduced fat milk (Kirkland), 35.7 ± 0.1 for Invitrogen fetal bovine serum, 22.1 ± 0.9 for human serum, 22.3 ± 0.2 for human plasma, 31.8 ± 1.1 for rat serum, 42.6 ± 0.1 for rat plasma and 1501 ± 52 for a fresh human urine sample, 0.21 ± 0.03 in a human BAL sample, 0.15 to 2.7 mg/dL in cell culture.

A scientific instrument known as a urea assay kit is used to determine the amount of urea present in a sample of blood or urine. When the body breaks down protein, urea is created in the liver as waste. The body then eliminates it through the kidneys in pee. Urine and blood urea levels can offer crucial diagnostic data about kidney function and other medical problems.

The assay kit usually comes with all the required materials and instructions for determining the sample's urea concentration. Typically, it transforms urea into ammonium ions, which can then be measured colorimetrically or fluorometrically. Depending on the particular assay kit, the kit may use various technologies and detection techniques.

Procedure for 96-well Plate 

1. Serum and plasma samples can be assayed directly (n = 1). Urine samples should be diluted 50-fold in distilled water prior to assay (n = 50). Transfer 5 µL water (blank), 5 µL standard (50 mg/dL) and 5 µL samples in duplicate into wells of a clear bottom 96-well plate. For low urea samples (< 5 mg/dL), e.g. tissue/cell extract, BAL etc, transfer 50 µL water (blank), 50 µL 5 mg urea/dL (the 50 mg/dL standard diluted 10 in water) and 50 µL samples in duplicate into separate wells. For culture medium containing phenol red, transfer 50 µL medium (blank), 50 µL 5 mg urea/dL (the 50 mg/dL standard diluted 10 in medium) and 50 µL samples in duplicate into separate wells.

2. Add 200 µL working reagent and tap lightly to mix.

3. Incubate 20 min (50 min for low urea samples) at room temperature.

4. Read optical density at 520 nm. For low urea samples, read OD at 430 nm.