Pregnancy and Urolithiasis Workup
- Author: Robert O Wayment, MD; Chief Editor: Bradley Fields Schwartz, DO, FACS more...
Laboratory studies in pregnant patients with possible urolithiasis include the following:
Urinalysis -To assess for microscopic hematuria
Serum chemistry studies, including a renal panel
Urine culture - To identify the offending bacteria and determine antibiotic sensitivities
Complete blood cell (CBC) count - To determine the presence or absence of systemic infection
Metabolic studies - To guide stone prevention
Consider the following:
The presence of red blood cells may suggest a calculus
Using both dipstick and microscopic analysis can identify microscopic hematuria in 95% of patients with urinary stones
Pyuria, which can result from an inflammatory reaction to a stone or infection, mandates evaluation of a coexisting urinary tract infection. Urinary tract infection is present in approximately 31% 
Urine pH greater than 7 may signal the presence of infected stones (magnesium-ammonium-phosphate) with urea-splitting organisms (eg, Proteus and Klebsiella species)
Acidic urine (pH < 5) suggests the presence of a uric acid stone
Decreased serum bicarbonate and potassium levels suggest an underlying renal tubular acidosis that may result in the formation of calcium phosphate stones. Elevated serum calcium levels might suggest possible primary or secondary hyperparathyroidism. Hyperuricemia suggests possible gouty diathesis and hyperuricosuria, which can increase the risk for both uric acid and calcium stone formation.
Elevated serum creatinine levels suggest azotemia due to ureteral obstruction or chronic renal insufficiency. The physiologic increase in glomerular filtration rate (GFR) during pregnancy dictates that the serum creatinine and BUN levels should be nearly 25% less than levels in the nonpregnant patient.
For patients with a sincere interest in reducing their risk of developing additional urinary stones, a 24-hour urine collection for stone prevention analysis is recommended. However, because of the physiologic and electrolytic changes associated with pregnancy, metabolic studies should be postponed until completion of pregnancy. Patients undergoing metabolic analysis studies should be willing to make long-term changes in their diet or lifestyle and take medications and/or supplements to help reduce their risk of new stone formation.
The metabolic evaluation should include, as a minimum, a 24-hour urine collection and determination of total volume and sodium, oxalate, citrate, uric acid, calcium, phosphate, and magnesium. Various commercial programs are available from laboratories such as Mission Pharmacal, LabCorp, Litholink, Dianon Systems, Nichols, and UroCor. All of these provide accurate and reliable results from the 24-hour urine collections.
The cornerstone of the metabolic evaluation is the stone analysis. All patients should be encouraged to strain urine until the stone passes or repeat imaging is performed.
Radiologic diagnosis of urolithiasis in pregnant patients is complicated by the physiologic and hemodynamic changes of pregnancy, such as increased renal blood flow (RBF) and GFR, in addition to the concern for fetal radiation exposure. Delay in diagnosis or inappropriate therapy may risk maternal renal damage, premature labor, spontaneous abortion, pyonephrosis, and/or maternal hypertension. Tailor the diagnostic evaluation and management of the gravid patient to the individual.
The use of ionizing radiation during pregnancy is a complicated and controversial issue. Radiographic studies should be used judiciously and avoided when possible, particularly during the first trimester. The guidelines available from the American College of Obstetricians and Gynecologists (ACOG) state, “concern about possible effects of high-dose ionizing radiation exposure should not prevent medically indicated diagnostic x-ray procedures from being performed on a pregnant woman. During pregnancy, other imaging procedures not associated with ionizing radiation (eg, ultrasonography, MRI) should be considered instead of x-rays when appropriate.” Concerning fetal dosages of radiation, ACOG states that “....less than 5 rads (5000 millirad [mrd]) has not been associated with an increase in fetal abnormalities or pregnancy loss.”
Srirangam et al (2008) reported average fetal radiation doses for common diagnostic procedures used in the workup of urolithiasis in pregnancy, as follows:
Ultrasonography - None
MRI (< 1.5 T) - None
KUB, 1.4 milligray (mGy) - 140 mrd
Intravenous urography, 1.7 mGy - 170 mrd
Renal tract CT scanning, 80 mGy - 800 mrd
Technetium Tc 99m renal scan (mercaptoacetyltriglycine [MAG-3] or diethylenetriaminepentaacetic acid [DTPA]), 0.2 mGy - 20 mrd
Renal ultrasonography, with or without Doppler studies, is recommended as the primary imaging modality in pregnant women. In the event that ultrasonography findings are equivocal and clinical symptoms strongly suggest renal calculi, a limited IVP with reduced films and radiation exposure may be performed. If the ultrasonography and limited IVP test findings are unclear, additional tests or procedures may be indicated, depending on the clinical scenario. However, when indicated, many suggest proceeding directly to ureteroscopy for diagnosis and treatment, especially in the first and second trimesters.[21, 22, 23] Radiation exposure in the third trimester is less of a risk to the fetus.
Renal ultrasonography is the first-line screening tool for urolithiasis in pregnant patients. Stothers and Lee (1992) found that renal ultrasonography provided a sensitivity of 34% and specificity of 86%, yet Parulkar et al (1998) reported 95% and 87%, respectively. The sonogram may not actually show the stone.
However, false-positive results may occur in the setting of extrarenal pelvis, vesicoureteral reflux, a high urine-flow rate, parapelvic cysts, and crossing vessels within the renal sinus. Up to 35% of patients with documented acute ureteral obstruction may not demonstrate any significant hydronephrosis, which makes standard ultrasonography less useful. Furthermore, differentiating hydronephrosis caused by an obstructing calculus due to physiologic dilation of pregnancy may be difficult.
Advantages of renal ultrasonography include avoidance of radiation exposure to the fetus, no pain, avoidance of proallergenic intravenous contrast material, and the ability to examine coexisting abdominal or pelvic disease etiologies.
This has been found valuable in revealing stones in the distal ureter that are not visualized with renal ultrasonography. Laing et al (1994) reported that distal ureteral stones were identified in 13 of 13 patients; renal ultrasonography revealed the distal stones in only 15% of the 13 patients. Laing et al also observed that patients tolerated the procedure well. Loughlin and Ker (2002) endorse the use of a transrectal ultrasonography probe if a vaginal transducer is unavailable.
Renal ultrasonography with Doppler sonography
In contrast to standard renal ultrasonography, ultrasonography with Doppler studies enables recording of waveform tracings of the renal vasculature. Ureteric obstruction increases renal vascular resistance, resulting in a reduction of diastolic blood flow and a rise in renal resistance. Based on waveform tracings, a resistive index (RI) value is calculated (RI = peak systolic velocity - peak diastolic velocity / peak systolic velocity), providing improved sensitivity and specificity for differentiating obstructed from nonobstructed dilated collecting systems. An elevated RI value of greater than 0.70 is specific for ureteral obstruction. Alternatively, a difference in the RI of 0.04 or more between the affected and contralateral kidney also suggests an obstruction in the side with the higher RI value.
Shokeir and Abdulmaaboud (1999) also evaluated the change in RI, which showed increased sensitivity (88%) and specificity (98%) in diagnosing ureteral obstruction.
Color Doppler renal sonography is a new addition to sonographic visualization of calculi, with a reported sensitivity of 100% and a specificity of 91% for diagnosing ureteral obstruction. This important study demonstrates the presence of ureteral jets (streams of densely opacified urine) flowing into the bladder (containing dilute nonopacified urine). The absence of these jets may suggest ureteral obstruction, while symmetric jets indicate the absence of obstruction. In addition, color Doppler studies also aid in differentiating iliac vessels from a dilated ureter.
Equivocal sonographic results that do not suggest either physiologic hydronephrosis of pregnancy or urolithiasis require further imaging with limited excretory urography.
Disadvantages of renal sonography include the following:
Suboptimal determination of the level of obstruction
Difficulty in showing the ureters and intraureteral calculi
Possible difficulty differentiating physiologic hydronephrosis of pregnancy from acute obstructive hydronephrosis
Unable (in most cases) to determine the size or shape of the urinary calculi
Difficulty visualizing calculi obscured by overlying bony structures, fetal skeleton, or fecal material
Disadvantages of renal sonography with color Doppler include the following:
Relies on elevated urine output and density differences between urine in the bladder and urine existing within the ureter
Degree of asymmetry of the ureteral jets unaltered from reference range because of calculi causing low-grade or no obstruction
Normal findings on renal sonography are consistent with the following results:
Degree of renal and ureteral dilation consistent with pregnancy
RI value of less than 0.70 in both kidneys
Symmetric ureteral jets
No specific calculus identified
The following results indicate a high probability of urolithiasis during pregnancy:
Greater degree of dilatation disproportionate to hydronephrosis of pregnancy in collecting system
RI value greater than 0.70 in the symptomatic kidney or change in RI greater than 0.60 
Dilated ureter extending below the level of the iliac arteries
Asymmetry of ureteral jets
Identification of calculus
Excretory urography remains an important diagnostic modality for stone detection in nonpregnant women, allowing the investigator to accomplish the following:
Establish the presence of an obstruction
Locate and determine the size of the offending calculus
Estimate renal function
Identify anatomic abnormalities that may alter the treatment algorithm
Detect altered renal physiology secondary to obstruction
Intravenous urography (IVU or IVP) consists of initial abdominal radiography of the kidneys, ureters, and bladder (KUB) followed by a second radiograph obtained 20-30 minutes after the intravenous injection of a contrast medium. The initial KUB radiograph exposes the fetus to 0.002 Gy; however, because the standard IVU necessitates four or five films, the patient may be exposed to a total of 0.004-0.01 Gy. The dose of radiation during IVU has been reported to be safe to the fetus during the second and third trimesters.
Limited IVP, however, has been shown to successfully reveal calculi without the high radiation dose of full IVP. Stothers and Lee (1992) recommend a scout film, a 30-second film, and a 20-minute film. They report successful visualization of 16 of 17 stones in pregnant patients who presented with acute renal colic. See the image below.
Indications for excretory urography in a pregnant patient may include the following:
Sonography results that are equivocal for pregnancy dilatation or urolithiasis
Azotemia suggestive of postrenal obstruction
Persistent fever or persistent positive finding on urine culture despite 48 hours of parenteral antibiotic treatment
Massive hydronephrosis on abdominal ultrasonography
Disadvantages of IVU include the following:
Risk of intravenous contrast allergy in the mother and fetus
Risk of radiation exposure to the mother and fetus
Possible ambiguous differentiation between delayed excretion of contrast material from calculus obstruction and pathologic hydronephrosis, especially in the third trimester
Small ureteral calculi obscured by enlarged uterus during IVU studies, especially in the third trimester
Magnetic resonance imaging
MRI provides high-quality images of the kidneys and urinary tract with obstruction and is used by some as second line to ultrasonography. Considerations are as follows:
MRI visualizes stones poorly; it cannot demonstrate the actual stone, only the point of obstruction
Using T2-weighted imaging, MRI urography can be used to differentiate a physiological upper tract dilatation from a pathologic ureterohydronephrosis during pregnancy and to ascertain whether the obstruction is intrinsic or extrinsic
MRI does not use ionizing radiation or iodinated contrast, but its use during the first trimester is not recommended because the effect of MRI on fetal development is not clear
MRI is expensive, uncomfortable for the pregnant patient, and often unavailable
Nuclear renal scan
Nuclear renal scan using technetium Tc 99m DTPA is an excellent study for objectively establishing the differential renal function and the efficiency of drainage of the dilated collecting system (washout times). DTPA is cleared almost exclusively by glomerular filtration. The rate of clearance provides an excellent estimate of GFR. However, nuclear studies do not allow visualization of stones and provide very limited illustration of anatomy. Differing opinions exist on its utility in the diagnosis of urolithiasis in pregnancy.[4, 17]
Interpretation is as follows:
A drainage half-time of 20 minutes or more indicates obstruction, whereas a drainage half-time of 10 minutes or less indicates nonobstruction
Washout or drainage half-times of 10-20 minutes are considered indeterminate
Unenhanced helical CT scanning is reported to be highly sensitive (96-97%) and specific (96-99%) and has become the criterion standard in the diagnosis of urinary calculi. It is also effective in differentiating calculi from tumors or blood clots.
Despite these benefits, this study has traditionally been avoided in pregnant patients. Recently, White et al (2007) reported on the use of low-dose CT scanning in pregnant patients with suspected urinary calculi, with an average radiation dose of only 705.5 mrd. They assert that this offers an acceptable degree of risk, and they advocate its use in the setting of refractory flank pain and inconclusive ultrasonographic results in pregnant patients. However, the use of CT scanning, or any study that involves radiation, should be avoided when possible, and patients should be counseled appropriately.
Ureteroscopy has emerged as a safe and efficient way to treat urolithiasis during pregnancy. Ulvik and associates (1995) have used ureteroscopy to successfully treat urolithiasis and consider it as a diagnostic procedure in difficult cases. Rigid or flexible ureteroscopes may be used, but Ulvik et al feel that flexible scopes may be better suited in diagnosis during pregnancy.
Rittenberg and Bagley reported the use of ureteroscopy for diagnosis with local anesthesia alone in 1988. Currently available ureteroscopes are small and may be used with minimal or no anesthesia.[21, 29] Lemos and coworkers (2002) feel that ureteroscopy used solely for diagnosis may be aggressive but agree that it can be used as a single modality for diagnosis and removal of ureteral calculi in pregnancy. Ureteroscopy offers clear-cut diagnosis, with direct visualization, as well as definitive therapy in the same encounter.
This can successfully reveal ureteral stones in cases with ambiguous sonography and IVU results. However, this study is performed only during stent placement because of the invasiveness of the examination, possible introduction of bacteria and risk of sepsis, and the need for radiation, sedation, and cystoscopy. Routine retrograde pyelography is not recommended for documentation of ureteral calculi in pregnant patients.
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