How to Use Urinalysis for the Physician Assistant Exam
In many Physician Assistant Exam questions about kidney disease, the urinalysis (UA) can guide you toward the right answer. Abnormalities of the urine, especially hematuria and proteinuria, are typical question fodder on the PANCE/PANRE. Understanding such urinary abnormalities is vital in differentiating among causes of nephritis as well as in analyzing causes of kidney disease.
In evaluating any kidney problem — whether it’s acute kidney failure, chronic kidney disease, hematuria, or proteinuria — understanding how to interpret a urinalysis is vital.
The urinalysis has two basic components: the dipstick and the examination of urine sediment under the microscope:
Dipstick test: A dipstick is a urine test strip. It contains chemicals that react with a urine sample. When you look for abnormalities on the dip, you look for chemicals that react with the strip testing positive.
Urine sediment examination: Abnormalities in the sediment include the presence of red cells, white cells, and/or different types of urinary casts. Urinary casts are a combination of a certain type of cell and Tamm-Horsfall protein, a protein made in the kidney tubules. For example, a red cell plus Tamm-Horsfall protein is a red cell cast, and a white cell plus Tamm-Horsfall protein is a white cell cast.
Here’s how to interpret the results of the urinalysis:
The urine dips positive for blood: Hematuria is present. Now consider whether red cells are present on the urinalysis microscopic:
If red cells are present on the urinalysis microscopic, the differential diagnosis is huge and includes malignancy (bladder or kidney cancer), a kidney stone, kidney infarction, infection, and glomerulonephritis (GN).
If the urine dips positive for blood but red cells are not present on the urinalysis microscopic, then consider the diseases associated with myoglobinuria.
The urine dips positive for protein: Proteinuria is present. The urine dip can be 1+ to 4+ for protein. Depending on the degree of proteinuria, this result can be a sign of diabetic nephropathy (if a person has diabetes), nephrotic syndrome, or glomerulonephritis.
If urinalysis is positive for both blood and red cells in addition to protein, then glomerulonephritis is in the differential diagnosis.
The urine dips positive for glucose and for protein: This is an early clue that the person has diabetic-related renal disease or diabetic nephropathy. The next step is testing for microalbuminuria, in which the kidney leaks small amounts of albumin into the urine.
The urine dips positive for nitrites and leukocyte esterase: You’re dealing with a urinary tract infection (UTI). The urine sediment should also show pyuria, a predominance of white cells. A urine culture of > 105 is diagnostic of a urinary tract infection. Note that unless the patient is pregnant, asymptomatic bacteriuria is never treated.
The urine dips positive for leukocyte esterase and nitrites as well as a count greater than 50 WBC/HPF (white blood cell count in high power field magnification): You’re likely dealing with a classic presentation of a pyelonephritis. This patient will present with fever and flank pain.
Here are some other notes on urinary abnormalities:
Hyaline casts can be a sign of dehydration. Another potential clue to dehydration is an elevated specific gravity. A reading of > 1.020 is a sign of dehydration, whereas 1.010 is close to normal.
Squamous epithelial cells are not a sign of kidney disease; they represent a contaminated urinary specimen. The best way to avoid getting these cells in the urine is to obtain a midstream urinary collection.
On a test, anytime you see muddy brown granular casts, think acute tubular necrosis (ATN).
If the urinalysis is positive for red cells, dysmorphic red cells, and/or red cell casts, think vasculitis and/or glomerulonephritis (GN).
Pyuria means having white cells in the urine. However, the WBC can mean different things, depending on the clinical scenario:
Think urinary tract infection (UTI) if the person has pyuria, the dip is positive for nitrites and leukocyte esterase, and the person has symptoms of dysuria and urinary frequency. Often, for a urinary tract infection to be present, more than 10,000 colonies of a causative organism need to be in the urine culture.
Think acute interstitial nephritis (AIN) if the person has pyuria but the urinalysis dips negative for nitrites and leukocyte esterase and if the urine is positive for eosinophils.
You are evaluating a 25-year-old woman who is admitted to the hospital with a fever of 38.3°C (101°F), tachycardia, and left-sided flank pain of 24 hours duration. Urinalysis is positive for 1+ blood and 2+ protein, and it’s positive for nitrites and leukocyte esterase as well. Microscopic evaluation reveals > 50 WBC/HPF. Which of the following is the most likely diagnosis?
(A) Acute glomerulonephritis
(B) Acute interstitial nephritis
(D) Renal infarction
(E) Diabetic nephropathy
The answer is Choice (C), pyelonephritis. The key to answering this question correctly is not only in the clinical presentation but also in the urinalysis. A finding of nitrites and leukocyte esterase on the urine dip as well as significant pyuria with a WBC > 50 in combination with the unilateral flank pain and fever is a classic presentation for pyelonephritis.
Acute glomerulonephritis, Choice (A), isn’t the right answer because it almost never presents with unilateral flank pain. Also, with acute glomerulonephritis, the predominant finding on urinalysis is hematuria, which can include red cells, red cell casts, and/or dysmorphic erythrocytes.
Acute interstitial nephritis (AIN), Choice (B), can present with a fever, but it usually doesn’t present with unilateral flank pain.
The classic presentation of an acute renal infarction, Choice (D), is an abrupt onset of unilateral flank pain. However, red cells are usually predominant in the urinalysis on microscopic examination. Other causes of unilateral flank pain include an acute hydronephrosis or a kidney stone.
With a kidney stone, you’d expect to see primarily red blood cells on examination of the urinary sediment. Diabetic nephropathy, Choice (E), can have proteinuria, but pyuria isn’t a standard finding in diabetic nephropathy.