Page 619 - Hematology_ Basic Principles and Practice ( PDFDrive )

528 Part V Red Blood Cells

TABLE Serum Cobalamin: False-Positive and False-Negative Diagnosing Folate Deficiency
39.3 Test Results
Falsely Low Serum Cobalamin in the Absence of True Cobalamin Deficiency When combined with a clinical picture of megaloblastic anemia and
Folate deficiency (one-third of patients) additional results of cobalamin levels, the serum folate concentration
is the cheapest and most useful initial biochemical test to diagnose
Multiple myeloma folate deficiency (see Table 39.2). The serum folate level is highly
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TCI deficiency sensitive to folate intake, and a single hospital meal may normalize it
Megadose vitamin C therapy in a patient with true folate deficiency. Rapidly developing nutritional
Falsely Raised Cobalamin Levels in the Presence of a True Deficiency a folate deficiency first leads to a decline in the serum folate level below
Cobalamin binders (TCI and II) increased (e.g., myeloproliferative normal (less than 2 ng/mL) in about 3 weeks; it is a sensitive indicator
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states, hepatomas, and fibrolamellar hepatic tumors) of negative folate balance. However, isolated reduction of serum
TCII-producing macrophages are activated (e.g., autoimmune diseases, folate level in the absence of megaloblastosis (i.e., false-positive result)
occurs in one-third of hospitalized patients with anorexia, after acute
monoblastic leukemias and lymphomas) alcohol consumption, during normal pregnancy, and in patients on
Release of cobalamin from hepatocytes (e.g., active liver disease) anticonvulsants ; unfortunately, these are the very groups at high risk
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High serum anti-IF antibody titer for folate deficiency and the people who exhibit low serum folate levels
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a Although a low serum cobalamin level is not synonymous with cobalamin when they become folate deficient. Conversely, in 25% to 50% of
deficiency, 5% of patients with true cobalamin deficiency have low-normal cases (predominantly alcoholics) with folate-deficient megaloblastosis,
cobalamin levels, a potentially serious problem because the patient’s underlying the serum folate levels may be below normal or borderline (2 to 4 ng/
cobalamin deficiency will progress if uncorrected. mL). The serum folate level alone should never dictate therapy. It
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IF, Intrinsic factor; TC, transcobalamin. is important to consider the clinical picture, peripheral smear, and
bone marrow morphology and also to rule out underlying cobalamin
deficiency.
Thus a serum cobalamin concentration is less than 300 pg/mL in
99% of patients with clinical hematologic or neurologic manifesta-
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tions of cobalamin deficiency, and a cobalamin level of more than prevalence of an associated folate deficiency among populations
300 pg/mL predicts folate deficiency or another hematologic or (mostly in developing countries, worldwide) where the dietary intake
neurologic disease (see Table 39.2). However, a low serum cobalamin of both vitamins is consistently low. Folate deficiency can also reduce
concentration is not synonymous with cobalamin deficiency, and serum cobalamin, but the mechanism is unclear.
several associated diseases and conditions can falsely raise or lower
cobalamin levels (Table 39.3). Studies have also identified patients
with true cobalamin deficiency who have cobalamin levels in the Serum Folate Levels
low-normal range. Among 173 unambiguously cobalamin-deficient
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patients about 5% had normal cobalamin levels. The serum folate level is clinically relevant and widely used. Micro-
If the serum cobalamin test is broadly used as a screening test biologic assays for folate, which measure all biologically active forms
without clinical context, by virtue of the way normalcy is defined, equally, have been replaced in the West by competitive folate-binding
2.5% of nondeficient individuals will have low levels, which reflects protein assays (from various commercial sources) that are indirect
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our definition of the lower limit of normal for this test. However, immunoassays, which rely on chemiluminescence methods. These
the finding that the same blood sample can give different cobalamin tests are notorious for considerable lack of agreement with one
results (one below normal versus one above normal) using different another (see box on Diagnosing Folate Deficiency). Alignment with
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commercial assays is of significant concern. The more recent assays a new higher-order precision isotope-dilution liquid chromatogra-
have periodically had such problems, apparently arising from a lack phy–tandem mass spectrometry assay, which demonstrates excellent
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of transparency related to these tests, poor validation using low- agreement with the traditional Lactobacillus casei method, will
cobalamin sera, and poor track record of continuous proficiency allow better standardization of the current competitive folate-binding
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testing and tracking of assay performance. It is a particularly serious protein assays. 121
issue when chemiluminescent tests for serum cobalamin give spuri- When negative folate balance continues, hepatic folate stores are
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ously elevated levels and fail to detect clinically significant severe depleted in about 4 months. This leads to tissue folate deficiency,
pernicious anemia. Such a false negative test result has been attributed which clinically correlates with a decrease in RBC folate (less than
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to the in vitro binding of anti-IF antibodies (that are found in the 150 ng/mL) by the microbiologic assay. However, current RBC
serum of a patient with pernicious anemia) to the intrinsic factor folate tests using different commercial kits have major limitations in
found in the manufacturer’s reagent. 117,118 However, a recent reevalu- sensitivity and specificity and are notoriously unreliable in alcoholics
ation of five different (currently used) automated cobalamin assays and in pregnancy; furthermore, a reduction of RBC folate also occurs
found that they are accurate and do not suffer from earlier prob- in about 60% of patients with cobalamin deficiency.
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lems. Nevertheless, the principle that “a clinical presentation which The use of red-cell folates as a measure of long-term folate status
strongly suggests cobalamin deficiency should always lead to a therapeutic is valid during clinical trials in which a single kit is used for a cohort
trial (with cobalamin replacement) even if the laboratory assay is non- of patients; however, it is not valuable for routine clinical diagnosis
concordant” must be upheld against future vagaries that can lead to because of the significant variability of performance between differ-
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dangerous false negative errors in laboratory tests. ent commercial kits and lack of clinical validation. For these
So in the absence of availability of metabolite tests, if there are reasons, the serum folate level, although labile, is a good initial
hematologic or neurologic findings that are consistent with clinical choice. 15,123–126
cobalamin deficiency, and the serum cobalamin level is normal or However, there are important caveats to measuring serum folate
borderline low, it is entirely appropriate to treat as for a cobalamin levels in certain clinical settings (see Caveats Related to the Use of
deficiency. If there is no improvement in hematologic parameters Laboratory Tests in Developing Countries). First, the serum folate
within a couple of months, provided there are no other conditions level can be artificially raised in a patient with either pure cobalamin
that limit a full response to cobalamin (e.g., iron deficiency or deficiency or combined cobalamin- and folate- deficiency (Table 39.4).
underlying thalassemia trait, hypothyroidism, renal disease, infection, This is because cessation of the cobalamin-dependent methionine
alcoholism, or intrinsic hematologic disease in the bone marrow), synthase reaction leads to a failure in utilization of intracellular folate
cobalamin deficiency would be unlikely. for one-carbon metabolism. As a result, folate leaks out of cells into
Cobalamin deficiency can falsely raise serum folate by 20% to the plasma, thereby raising the patient’s serum folate level; indeed,
30% via methyl-folate trapping. This will seriously underestimate the replacement of cobalamin alone will return the serum folate level