Urea and Electrolytes and eGFR

Urea and electrolytes

Urea and electrolytes (U&Es) is a commonly performed blood test that usually measures sodium, potassium, urea, creatinine, and the estimated glomerular filtration rate (eGFR).

Estimated Glomerular Filtration Rate (eGFR)

The estimated glomerular filtration rate (eGFR) is used to measure kidney function. Creatinine is made by skeletal muscle and excreted in the kidneys. Using creatinine alone may not be useful for assessing kidney function, hence the eGFR is used. Its calculation is based on serum creatinine and factors in age, sex, and ethnicity.

Urinary Albumin:Creatinine Ratio

Overview

Urinary albumin:creatinine ratio (ACR) is used to evaluate proteinuria. It is also used to screen patients with chronic kidney disease (CKD) or those who are at an increased risk for its development.

It involves collecting a spot sample, making it more convenient than collecting a 24-hour urine sample. For results to be reliable, the sample should be a first-pass morning urine specimen.

Urine Cytology

Overview

Urine cytology involves a microscopic examination of cells shed from the urinary tract. It may assist in the diagnosis of urinary tract cancers, such as bladder cancer. However, it should not replace cystoscopy.

CT Urogram

Overview

A computed tomography urogram (CTU) can be used to visualise the upper urinary tract. Ideally, it is performed in 3 overall steps:

  • Non-contrast phase – a CT of the kidneys, ureter, and bladder is performed:
  • Contrast phase – contrast is given and an image is taken while it passes through the kidneys:
    • This can show tissue changes and can identify tumours
  • Excretory phase – a final image is taken as the contrast is passed in the urine:
    • This can identify filling defects in the kidney, ureter, or bladder and may identify urothelial tumours

Multiparametric MRI

Overview

A multiparametric MRI (mpMRI) provides more detailed images of the prostate gland compared to standard MRI images. It involves combining 4 different types of images. During the scan, a gadolinium–based contrast agent is given.

Micturating Cystourethrogram (MCUG Scan)

This is used to assess the kidney and bladder using a radiopaque contrast and x-ray imaging. It is useful in the assessment of urinary reflux. Contrast is administered via a urinary catheter and the patient is then asked to void. Images are then taken as the contrast is passed.

Renal Biopsy

A renal biopsy is often taken with imaging. This can be done percutaneously (inserting a needle through the skin) or via a transjugular route. Once a specimen is obtained, it is analysed using microscopy:

  • Light microscopy:
    • Can identify general inflammation, immune complex deposits, renal tissue cell numbers, and the characteristics of the basement membrane
  • Immunofluorescence microscopy:
    • Involves adding antibodies labelled with a fluorescent marker to the tissue sample and can identify specific immunoglobulins (e.g. IgA and anti-glomerular basement membrane antibodies), complement proteins, and light chains in multiple myeloma.
  • Electron microscopy:
    • Can identify extremely small abnormalities, such as the loss of podocytes in minimal change disease, which are too small to see with light microscopy.

Ureteroscopy and Cystoscopy

Ureteroscopy

Ureteroscopy is where an endoscope (ureteroscope) is passed through the urethra and the bladder into the ureter. This may be helpful in the identification and management of renal stones and cancers of the upper urinary tract.

Cystoscopy

Cystoscopy is where an endoscope is passed through the urethra to visualise the bladder.

Urinalysis

Overview

In general, urinalysis describes the assessment of urine via inspection, the use of a urine dipstick, and microscopic evaluation.

Inspection

The following factors are assessed:

  • The colour of the urine:
    • Pale, straw-coloured urine – normal
    • Dark, yellow urine – suggests dehydration
    • Red urine – suggests blood
    • Brown urine – suggests bilirubin or myoglobin
  • The turbidity of the urine:
    • Clear urine – normal
    • Cloudy urine – may suggest a urinary tract infection (UTI) or proteinuria
    • Frothy urine – suggests proteinuria
  • Sometimes the odour is assessed – this is rarely done:
    • Offensive odour – suggests UTI
    • Sweet odour – suggests glycosuria

Urine dipstick analysis

A urine dipstick is inserted into a urine sample. Generally, on the side of the dipstick container is a guide for interpreting the readings, which are all read off at different times. Common factors measured are:

  • Blood – see Haematuria for more information
  • Protein – may suggest proteinuria (e.g. chronic kidney disease and nephrotic syndrome)
  • Nitrites – suggests bacterial infection if positive
  • Leukocyte esterase – suggests the presence of white cells if positive
  • Glucose – suggests glycosuria if positive (e.g. diabetes mellitus)
  • Bilirubin – suggests bilirubinuria (e.g. jaundice)
  • Ketones – suggests ketonuria (e.g. diabetic ketoacidosis)
  • Specific gravity – measures the amount of solute dissolved in the urine
    • Low specific gravity suggests dilute urine (e.g. diabetes insipidus, excessive fluid intake)
    • High specific gravity suggests concentrated urine (e.g. dehydration and glycosuria)
  • pH:
    • Low pH suggests diabetic ketoacidosis, sepsis, or other causes of metabolic acidosis
    • High pH suggests conditions causing metabolic alkalosis, such as vomiting
  • Urobilinogen – a product of bilirubin breakdown normally excreted in the urine

Urine microscopy

Urine microscopy is generally performed if abnormalities are present on a dipstick or patients have features that may warrant a microscopic evaluation. Microscopy may identify the presence of:

  • Casts:
    • Red cell casts – suggests acute tubular necrosis, glomerulonephritis, intrinsic renal disease
    • White cell casts – suggests pyelonephritis, interstitial nephritis, and renal inflammation
    • Epithelial cell casts – suggest acute tubular necrosis, interstitial nephritis, glomerulonephritis, eclampsia, nephritic syndrome, and transplant rejection
    • Muddy brown casts – suggest acute tubular necrosis
  • Cells:
    • Red cells – associated with causes of haematuria
    • White cells – suggests UTI or inflammation
    • Tubular cells – suggests acute tubular necrosis
    • Bacteria, fungi, or parasites may be present
  • Crystals:
    • Uric acid – suggests uric acid nephropathy
    • Calcium oxalate – may normally be seen or suggests ethylene glycol poisoning
    • Cysteine – suggests cystinuria

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