Written by ◆ Yang Qianni and compiled by ◆ Liang Yingxiu
Urine test strips are a commonly used hematuria screening tool. The sensitivity is comparable to urine microscopy, but there will be more false positive results because it cannot distinguish whether the urine occult blood reaction is caused by red blood cells, heme or myoglobin. .
However, false negative results from urine dipsticks are uncommon and can usually reliably rule out abnormal hematuria. When the urine test paper result is positive, further urine microscopy is required to confirm or exclude false positive results.
(Kuala Lumpur News) The so-called hematuria refers to an increase in the number of red blood cells in the urine. Hematuria can be “clearly visible” red or brown urine, which is called gross haematuria or macroscopic hematuria, while what is visible under a urine microscope is called microscopic haematuria ).
However, red or brown urine does not necessarily mean hematuria, because some drugs or foods can also cause changes in urine color. Common drugs include the antibiotics rifampin, nitrofurantoin, and metronidazole. ), as well as the diuretic triamterene, the anesthetic propofol and the laxative senna, etc.
Menstrual sex affects outcomes
Dr Selva Kumar Sivapunniam, Consultant Pediatric Nephrology, pointed out that foods in daily life that can cause red or brown urine include foods containing artificial colorings, beets, blackberries, rhubarb and broad beans, etc. . “In addition, conditions such as hemoglobinuria (hemolytic anemia) or myoglobinuria (muscle damage) can also cause red urine.”
“In fact, everyone’s urine contains red blood cells. Normally, about 66,000 red blood cells are excreted in 12 hours. In comparison, a patient with glomerulonephritis can excrete about 66,000 red blood cells in 12 hours. 40 million to 120 million red blood cells. When the red blood cell count in the 12-hour urine sediment exceeds 500,000, it indicates an abnormal increase in red blood cells in the urine.
According to this, the definition of hematuria is equivalent to the discovery of 2 or more red blood cells under high power field examination. However, certain conditions such as recent strenuous exercise, menstruation, sexual behavior, catheter insertion, etc. can affect the results. “
Microhematuria can be distinguished based on the presence or absence of symptoms. Asymptomatic microhematuria means not associated with pain (dysuria, low back pain, renal colic), abnormal renal function, hypertension or proteinuria. Microhematuria is common in children, especially isolated asymptomatic microhematuria, which is mostly benign. Therefore, the challenge for pediatricians is to identify whether a child’s hematuria is caused by a specific underlying disease and to avoid unnecessary testing of children with benign hematuria.
“According to surveys, the prevalence of hematuria in school-age children obtained from a single urine test is about 3% to 4%.
After repeating 2 or more urine tests, the prevalence dropped to 1%.
Among the 1% of children with two or more positive hematuria,
One-third had persistent hematuria, defined as a repeat urine test that was positive after 6 months.
As for the combination of hematuria and proteinuria, which is less common,
The prevalence from a single urine test among school-age children is less than 0.7%. “
He said that microhematuria in children can be temporary or persistent. The former can be caused by strenuous exercise, fever, kidney or urinary tract trauma, urinary tract infection and urethritis. Common causes of the latter include glomerulopathy. (glomerulopathies), hypercalciuria, and Nutcracker Syndrome (NCS, also known as left renal vein compression syndrome).
Urolithiasis causing non-renal hematuria
According to the source of bleeding, it can be clinically divided into glomerular hematuria, non-glomerular renal hematuria, and non-renal hematuria. Nephritis caused by infection, immunoglobulin type A nephropathy (IgA nephropathy), Henoch-Schonlein purpura (HSP), hereditary nephritis, benign familial hematuria, membranoproliferative glomerulonephritis (MPGN) and lupus erythematosus nephritis are caused by Common causes of glomerular hematuria.
“Hematuria from other structures of the kidney other than the glomerulus can be caused by nephrocalcinosis, interstitial nephritis, pyelonephritis, renal vein thrombosis, sickle hemoglobin (HbS), Caused by malformations (aneurysms, hemangiomas, and arteriovenous fistulas), Wilm’s tumor (malignant renal tumor), renal cell carcinoma, and cystic diseases (polycystic disease, simple cysts).
He pointed out that causes of non-renal hematuria include urolithiasis, hypercalciuria, papillary necrosis (caused by infection, diabetes or HbS, etc.), trauma, bacterial or viral infection, drugs such as cyclophosphamide, prostatitis, strenuous exercise, vascular malformations, and urinary tract foreign bodies.
“Causes of neonatal hematuria include renal vein or arterial thrombosis, autosomal recessive polycystic kidney disease, obstructive urinary tract disease, urinary tract infection, bleeding and coagulation disorders, and trauma such as bladder catheterization.”
Sensitivity 93% to 100% Urine test strips have many false positive results
The two most important diagnostic methods for hematuria in children are urine dipstick and urine microscopy (urine microscopy for short).
Urine test strips are the most common hematuria screening tool, and their sensitivity is comparable to urine microscopy, ranging from 93% to 100%. However, there will be more false positive results because it cannot distinguish whether the urinary occult blood reaction is caused by red blood cells, heme, or myoglobin. However, false negative results from urine dipsticks are uncommon and can usually reliably rule out abnormal hematuria.
Urine microscopy to rule out false positive results
Sivakumar pointed out that urine test strips use hydrogen peroxide to catalyze the chemical reaction between hemoglobin (or myoglobin) and the color developer Tetramethylbenzidine, and then display different shades. color.
Urine test strips can detect 5 to 10 intact red blood cells per microliter, which roughly corresponds to the result of 2 to 5 red blood cells in a centrifuged urine sample of 10 to 15 ml under high-power microscopy.
It should be noted that formalin used as a preservative or high concentrations of vitamin C may cause false negative results.
False-positive results may occur in urine that is alkaline (pH greater than 9) or contaminated with oxidants used to clean the perineum.
When the urine test paper result is positive, further urine microscopy is required to confirm or exclude false positive results; urine microscopy is the gold standard for detecting microscopic hematuria.
The sample used for urine microscopy should be between 10 and 15 ml of fresh urine to limit red blood cell lysis (or hemolysis) and false negative results.
Microhematuria is defined as the presence of more than 5 red blood cells per high-power field (magnification 40 times).
Identify the source of bleeding to aid subsequent diagnosis
Urine microscopy can identify potential sources of hematuria, such as nephroglomerular hematuria, and help identify the underlying cause. When the urine sediment contains red blood cell cylinders (RBC cast) or deformed red blood cells (dysmorphic RBC), it is suggested that the cause of hematuria may be glomerulonephritis.
Identification of the source of bleeding is important for patient prognosis and subsequent diagnostic evaluation. In addition, when asking about the medical history, it is necessary to exclude exogenous blood such as menstrual blood, as well as special conditions such as Munchausen syndrome (Munchausen syndrome).