Page 15 - Acute Pancreatitis (Viêm tụy cấp)

906 PART VII Pancreas

is known as Purtscher retinopathy and can be seen in a variety of salivary glands, fallopian tubes). Furthermore, mass lesions such
conditions besides AP. 180 as papillary cystadenocarcinoma of the ovary, benign ovarian cyst,
and carcinoma of the lung can cause hyperamylasemia because
DIFFERENTIAL DIAGNOSIS they produce and secrete salivary (S-type) isoamylase. Leakage
of P-type isoamylase across the intestine with peritoneal amy-
The abdominal pain of biliary pain may simulate AP. It is fre- lase absorption probably explains hyperamylasemia in patients
quently severe and epigastric, but it typically lasts for several with intestinal infarction or GI tract perforation. Renal failure
hours rather than several days (see Chapter 65). The pain of a per- can increase serum amylase up to 4 to 5 times the upper limit of
forated peptic ulcer is sudden, becomes diffuse, and precipitates a normal because of decreased renal clearance of this enzyme. 185
rigid abdomen; movement aggravates pain. Nausea and vomiting Patients on hemodialysis tend to have higher serum amylase lev-
occur but disappear soon after onset of pain (see Chapter 53). els than those on peritoneal dialysis. In patients with chronic kid-
In mesenteric ischemia or infarction, the clinical setting often is ney disease, there is not a clear inverse correlation between the
an older person with atrial fibrillation or atherosclerotic disease creatinine clearance rate and serum levels of amylase, and about
who develops sudden pain out of proportion to physical findings, one third of patients with marked renal insufficiency (low creati-
bloody diarrhea, nausea, and vomiting. Abdominal tenderness nine clearance) have normal pancreatic enzyme levels.
may be mild to moderate, and muscular rigidity may not be severe Chronic elevations of serum amylase (without amylasuria)
despite severe pain (see Chapter 118). In intestinal obstruction, occur in macroamylasemia. In this condition, normal serum amy-
pain is cyclical, abdominal distention is prominent, vomiting per- lase is bound to an immunoglobulin or abnormal serum protein
sists and may become feculent, and peristalsis is hyperactive and to form a complex that is too large to be filtered by renal glom-
often audible (see Chapter 123). Other conditions that enter into eruli and thus has a prolonged serum half-life. 185 Macroamyla-
the differential diagnosis of AP are listed in Box 58.6. semia may lead to a false diagnosis of pancreatic disease, but it
has no other clinical consequence. The urinary amylase-to-cre-
LABORATORY DIAGNOSIS atinine clearance ratio (ACCR) increases from approximately 3%
to approximately 10% in AP. 186 However, even moderate renal
Pancreatic Enzymes insufficiency interferes with the accuracy and specificity of the
ACCR. Other than to diagnose macroamylasemia, which has a
In general, the diagnosis of AP relies on at least a 3-fold elevation low ACCR, urinary amylase measurements and the ACCR are
of serum amylase or lipase in the blood. 181 not used clinically. Macroamylasemia can also be measured
directly in serum samples. Deliberate contamination of urine
Serum Amylase Level with saliva, as in Munchausen syndrome, can increase the urine
amylase, with the serum amylase being normal. This situation can
In healthy persons, the pancreas accounts for 40% to 45% of be excluded by measuring S-type amylase in the urine.
serum amylase activity, the salivary glands accounting for the In the emergency room, computer order set de-selection of
rest. Simple analytic techniques can separate pancreatic and sali- amylase but using lipase was an effective tool to reduce non-value-
vary amylases. Because pancreatic diseases increase serum pan- added testing and reduce cost while maintaining quality patient
creatic (P) isoamylase, measurement of P-isoamylase can improve care and physician choice in patients presenting with abdominal
diagnostic accuracy. However, this test is rarely used. pain. 187 The rapid and easy-to-operate amylase assay may have
The total serum amylase test is most frequently ordered to potential application in the fields of point-of-care clinical diag-
diagnose AP, because it can be measured quickly and cheaply. nosis, particularly in rural and remote areas where lab equipment
It rises within 6 to 12 hours of onset and is cleared fairly rapidly may be limited. 188
from the blood (half-life, 10 hours). Probably less than 25% of
serum amylase is removed by the kidneys. It is uncertain what Serum Lipase Level
other processes clear amylase from the circulation. The serum
amylase is usually increased on the first day of symptoms, and it The sensitivity of serum lipase for the diagnosis of AP is similar
remains elevated for 3 to 5 days in uncomplicated attacks. Sen- to that of serum amylase and is at least 85%. 181 Lipase may have
sitivity is at least 85%. The serum amylase may be normal or greater specificity for pancreatitis than amylase, however. Serum
6
only minimally elevated in fatal pancreatitis, during a mild attack lipase is normal when serum amylase is elevated in nonpancre-
or an attack superimposed on chronic pancreatitis (because the atic conditions such as salivary gland disease, amylase-producing
pancreas has little remaining acinar tissue), or during recovery tumors, gynecologic conditions such as salpingitis, and mac-
from AP as amylase is cleared from the circulation. The level may roamylasemia. Serum lipase always is elevated on the first day of
return to normal quickly, in just a few days. Serum amylase also illness and remains elevated longer than does the serum amylase,
may be falsely normal in hypertriglyceridemia-associated pan- providing a slightly higher sensitivity. 189 Combining amylase and
creatitis, 182 because an amylase inhibitor may be associated with lipase does not improve diagnostic accuracy and increases cost.
TG elevations. In this case, serial dilution of serum often reveals Specificity of lipase can suffer from some of the same prob-
an elevated serum amylase. Hyperamylasemia is also not specific lems as those of amylase, however. In the absence of pancreatitis,
for pancreatitis; it occurs in many conditions. In fact, one half of serum lipase may increase less than 2-fold above normal in renal
all patients with an elevated serum amylase level may not have insufficiency. 190 With acute GI conditions that resemble AP, 191
pancreatic disease. 181 In AP, the serum amylase concentration is serum lipase increases to levels less than 3-fold above normal,
usually more than 2 to 3 times the upper limit of normal; it is presumably by absorption through an ischemic, inflamed, or per-
usually less than this with other causes of hyperamylasemia. 183 forated intestine. Rarely, a nonpancreatic abdominal condition
However, this level is not an absolute discriminator. Thus an such as small bowel obstruction can raise the serum lipase (and
increased serum amylase level supports rather than confirms the amylase) above 3 times normal. Some believe that serum lipase
diagnosis of AP. In addition, there are some individuals who have measurement is preferable to that of serum amylase because it is
persistent hyperamylasemia without clinical symptoms. This has as least as sensitive as amylase measurement and more specific,
been reported to be due to macroamylasemia (discussed later) whereas others find no clear advantage of one over the other. 9
or pancreatic hyperamylasemia on a familial basis. 184 Nonpan- Many normal persons have elevations of serum amylase and/
creatic diseases that lead to hyperamylasemia include pathologic or lipase of little clinical significance. 192 Diabetics appear to have
processes in other organs that normally produce amylase (e.g., higher median lipase compared with nondiabetic patients for