A patient with renal disease can present either as an initial outpatient or inpatient consultation. Some patients may be referred because of abnormal urinary findings, such as hematuria or proteinuria, which may have been incidentally discovered during routine clinical evaluation or as part of initial employment requirements. Depending on the stage of renal disease, they can present with mild edema or generalized pruritus, as well as more advanced signs and symptoms of uremia, such as decreased appetite, weight loss, and even alterations in mental status. In general, the symptoms and signs of patients with renal disease tend to be nonspecific (Table 1–1). Still others would present only with elevation in serum creatinine.

To narrow the differential diagnosis, it is necessary to first determine whether the disease is acute, subacute, or chronic on presentation. However, there is usually an overlap in these stages, and at times, it is not exactly clear. Certainly, a patient who presents with an elevated serum creatinine that was documented to be normal a few days previously has an acute presentation, whereas a patient who presents with a previously elevated serum creatinine that has been rising steadily over the past several months to years has a chronic disease. Oftentimes, acute exacerbations of chronic renal disease are common presentations.

The next question concerns which segment or component of the renal anatomy is involved. This is subdivided into prerenal, postrenal, or renal (Table 1–2).

Prerenal disease refers to any process that decreases renal perfusion, such as intravascular volume depletion, hypotension, massive blood loss, or third spacing of fluids. It can also be due to congestive heart failure, whereby decreased effective circulating volume decreases blood flow toward the kidneys (see Chapter 9).

Postrenal disease refers to any obstruction that impedes urinary flow through the urinary tract. Examples include benign prostatic hypertrophy or cervical malignancy (see Chapter 16).

Renal involvement is further subdivided into vascular, glomerular (see Chapters 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, and 35), or tubulointerstitial disease (see Chapters 36 and 37), depending on which segment is involved.

The most common method of assessing renal function is by estimation of the glomerular filtration rate (GFR). The GFR gives an approximation of the degree of renal function. Daily GFR in normal subjects is in the range of 150–250 L/24 hours or 100–120 mL/minute/1.73 m2 of body surface area. GFR is decreased in those with renal dysfunction, and is used to monitor renal function in those with chronic kidney disease. It is also used to determine the appropriate timing for initiation of renal replacement therapy.

To date, there are several methods by which GFR is measured, namely serum creatinine concentration, 24-hour creatinine clearance, as well as estimation equations such as the Cockroft–Gault formula and the Modification of Diet in Renal Disease (MDRD) Study formula (Table 1–3).

Using the serum creatinine alone to estimate renal functioning is inaccurate for several reasons. First, a small amount of creatinine is normally secreted by the tubules, and this amount tends to increase as progressive renal decline occurs, thereby overestimating the true GFR value. Similarly, there are factors that increase serum creatinine without truly affecting renal function, such as dietary meat (protein) intake, volume of muscle mass, and medications that interfere with tubular secretion of creatinine such as cimetidine, trimethoprim, and probenecid. Elderly patients, those with cachexia, amputees, as well as patients with spinal cord injury or disease tend to have less muscle mass, hence, lower serum creatinine values (Table 1–4).