Damage to any part of the barrier system can lead to proteinuria. This can result from chronic kidney diseases such as diabetes. Other nephrotic syndromes, including minimal change disease and membranous nephropathy, and lupus also affect the glomerular basement membrane (GBM) and epithelial podocytes and cause proteinuria as well. Proteinuria may also result from acute injury to the glomerular capillary endothelial cell, which occurs in preeclampsia.
How to evaluate proteinuria
Ideally, clinicians should know if their patients have been proteinuric before pregnancy or whether this finding is new. Often, the only way to know if a preexisting problem existed is to track down old urinalysis results from primary-care physicians. Detecting new onset proteinuria can be done by the methods outlined.
Urine dipstick testing
This is the best method for in-office screening of pregnant patients for proteinuria. Obtain a fresh, midstream specimen before a vaginal exam is done. A positive dipstick reaction (1+) usually occurs once the protein concentration reaches a threshold of 30 mg/dL (Table 2). Although this is a good screening test, don’t use it to quantify the amount of proteinuria because it is prone to false-positive and false-negative results (Table 3). Be mindful of these situations so that you don’t alarm patients.
The most common false-positive results come from a highly concentrated urine (high specific gravity) in a patient not adequately hydrated. False negatives may result because the urine dipstick only detects albumin in the urine and not other proteins such as immunoglobulins that may occur in diseases like multiple myeloma. If you obtain a low level reaction (1+) on a dipstick, confirm it by repeated morning testing before progressing to more evaluation.
24-hour Urine collection
A 24-hour collection of urine has been the gold standard for quantifying proteinuria during pregnancy. It also allows the clinician to quantify protein excretion to measure creatinine clearance for evaluating kidney function.
24-hour collections have their limitations, however, and are only useful if the patient manages to collect a complete sample. Unfortunately only 10% to 20% of nonpregnant patients succeed in a complete collection.6 One can only imagine how much more difficult it is for pregnant patients because of increased urinary frequency and physical limitations to collection. The other major disadvantage to a 24-hour collection is that it can delay the diagnosis of critical problems like preeclampsia.
Because of the awkwardness of collection, the concern over incomplete results, and the time delay to diagnosis, many clinicians use a spot urine test noted below. It is a good screening test and can be used to follow proteinuria. Nevertheless, the 24-hour Urine collection remains the gold standard if done correctly.