How Is Chronic Kidney Disease Diagnosed?
U/A: The initial test in evaluating the kidney is the urinalysis (U/A). It can assess
- Turbidity: Cloudiness, indicating particulate matter, such as white blood cells (pus), red blood cells, epithelial cells, uric acid and other crystals, sediment, tubular casts (tubule-shaped clumps of white blood cells or red blood cells), and bacteria;
- Color: Dark due to dehydration or because of myoglobin (filtered debris of muscle damage); red or brown urine (tinged with blood); and
- Biochemical markers via dipstick: A mini-battery of tests for the presence of glucose, blood, nitrite, protein, pH, and specific gravity (how much the weight differs from plain water).
Microscopic evaluation is done with a spun-down urine to examine the sediment:
- Crystals: Uric acid (suggestive of acute kidney injury/tumor), calcium, cystine, and magnesium ammonium phosphate (consistent with infection)
- Microorganisms: Bacteria, fungi
- Cells: red blood cells (hematuria), white blood cells (pyuria), and epithelial or tumor cells
Red blood cells (RBCs) are normally absent in the urine, but when present can represent benign temporary effects from exercise, sexual intercourse, or menstrual contamination; alternately, hematuria could be the first evidence of malignancy, especially in the elderly. Red blood cells are also associated with intrinsic kidney disease, such as glomerulonephritis (inflammation of the nephron), or systemic disease, such as sickle cell anemia or systemic anemia from hemolysis (RBC destruction). The mechanical shearing of delicate urinary tract tissue from stone migration can cause microscopic hematuria.
White blood cells (WBCs) should not be present and when present indicate infection until ruled out (via a urine culture). In non-infectious pyuria (“sterile pyuria”), kidney stones or intrinsic kidney disease (e.g., nephritis) is considered.
- Casts: Cylinders of compressed structures that have loosened from the tubules and passed intact into the urine. When they are primarily made of protein, urinary acidity (low pH) or high urine concentration causes them.
RBC casts indicate intrinsic disease in the kidney, such as glomeruloneprhitis.
WBC casts indicate inflammation and infection, such as pyelonephritis infection.
GFR: Glomerular filtration rate is used to classify kidney damage and dysfunction, assessed as filtered blood per minute as applied to body size/area, i.e, mL/min/1.73 m2.
- Normal GFR is 125
- Chronic kidney disease is a GFR < 60
- Kidney failure is a GFR or < 15
Proteinuria/Albuminuria: chronic kidney disease (CKD) can be described in terms of abnormal amounts of protein spilled into the urine from filtration abnormalities; normal filtration is designed to keep albumin in the blood by way of limiting its excretion via glomerular permeability. When the permeability is abnormal, too much is excreted, and this can be determined by urinalysis.
Serum creatinine: elevated in those with CKD, even when they have no symptoms that would otherwise provoke suspicion. For this reason, the creatinine level is included in the routine metabolic profile blood test, which along with a complete blood count (CBC), is the universal starting point of screening in primary care. An albumin/creatinine ratio allows meaningful integration of these two components that increases accuracy of determining the degree of kidney dysfunction.
- Ultrasound can reveal small-sized kidneys, indicative of long-term scarring from infection. Structural anatomy, as is abnormal in congenital polycystic kidney disease, congenital absence of a kidney, or misplacement as a “pelvic” kidney can be identified.
- Kidney biopsy under ultrasound guidance: The position of the kidneys makes them very accessible to biopsy with just a needle insertion through the skin, visualized in real time with bedside ultrasound. A biopsy is done for an exact diagnosis, to judge the efficacy of treatment, to separate acute kidney disease or injury from chronic kidney disease, and to diagnose genetic disease.
- CT and MRI are also useful when other imaging is inconclusive.
- X-rays are all that are needed to identify a stone obstruction.
- Intravenous pyelogram: The injection of dye into the blood that is cleared through the urine, can light up the entire urinary tract to identify congenital anomalies, obstruction, or physical distortions to the kidneys, ureters, or bladder.