DLIS Concentrations Healthy Individuals versus Disease 231 Healthy Individuals

DLIS concentration in sera of subjects not taking any digoxin depends on the particular immunoassay used. The FPIA marketed by the Abbott Laboratories has significant cross-reactivity with DLIS. However, even with the FPIA, the concentrations of DLIS in healthy individuals are usually below the detection limit of the instrument (<0.20ng/mL) in most cases. One of the factors that may contribute to significant interference of the FPIA with DLIS is the use of a rabbit polyclonal antibody against digoxin in the assay design of FPIA. Newer digoxin immunoassays, which utilize a more specific monoclonal antibody against digoxin, are subjected to significantly less interference with DLIS.

2.3.2. Volume Expansion

Volume expansion is a major cause of elevated DLIS in blood. Elevated concentrations of DLIS have been reported in uremia, essential hypertension, hypertension of water volume expansion, liver disease, preeclampsia, liver and kidney transplant, congestive heart failure, premature babies, and other conditions (19-24).

2.3.3. Critically Ill Patients

Howarth et al. reported elevated DLIS in plasma of intensive care unit patients. Although some patients showed either hepatic or renal dysfunction, another 42 patients who showed elevated DLIS had neither hepatic nor renal dysfunction. The authors used an FPIA for measuring DLIS. The DLIS concentrations ranged from 0.0 to 1.69 nmol/L in 16 patients with coexisting hepatic and renal dysfunction, whereas 38 patients with hepatic dysfunction but normal renal function showed a range of DLIS concentration of 0.0-0.77 nmol/L. Four patients with renal dysfunction only had DLIS concentrations between 0.0 and 0.34 nmol/L, and the remaining 42 patients had DLIS concentrations ranging from 0.0 to 36 nmol/L (25).

Berendes et al. reported that different types of endogenous glycosides are elevated in significant proportions in critically ill patients. The authors used FPIA for digoxin and digitoxin for measuring DLIS in patients not treated with cardiac glycosides. Of the 401 critically ill patients, 343 (85.5%) did not show any measurable concentration of DLIS but the remaining 58 patients (14.5%) had measurable DLIS. Of these 58 patients, 18 patients showed significant digoxin levels (0.54 ± 0.36ng/mL) and 34 patients showed measurable digitoxin levels (2.28 ± 1.7ng/mL). Interestingly, mean endogenous ouabain concentrations were ninefold increased in DLIS-positive patients and only threefold increases were observed in DLIS-negative patients. The mortality of DLIS-positive patients was 12% whereas mortality in DLIS-negative patients was only 3.2% (26).

2.3.4. Pediatric Population

Concentration of DLIS may be significantly increased in cord blood as well as in sera of neonates. Chicella et al. measured DLIS concentrations in 80 pediatric patients never exposed to digoxin by using both FPIA and microparticle enzyme immunoassay (MEIA). Both digoxin assays are marketed by the Abbott Laboratories. The authors reported that 48% of the specimens showed measurable DLIS using the MEIA and 79% of the specimens showed measurable DLIS using FPIA, whereas values obtained by the FPIA were higher than the corresponding values obtained by the MEIA. The highest apparent digoxin concentration was 0.38ng/mL, and a poor correlation was noted between patient age, serum creatinine, total bilirubin, and DLIS concentration (27). Ijiri et al. reported that although DLIS concentrations were elevated in neonates with jaundice (0.58 ± 0.13ng/mL before phototherapy and 0.33 ± 0.06 ng/mL after phototherapy) compared with that in neonates without jaundice (0.34 ± 0.04 ng/mL), a fluorescent compound related to bilirubin increased the blank intensity measurement in the FPIA. This compound was not related to DLIS (28).

Concentrations of DLIS in cord blood may be significantly elevated compared with that in maternal blood. In one study, the mean DLIS concentration in umbilical cord plasma was 0.55 ng/mL whereas the average DLIS concentration in maternal plasma was 0.23 ng/mL (measured using FPIA). Moreover, dehydroepiandrosterone sulfate in maternal plasma and progesterone in maternal and umbilical cord plasma may be measured as digoxin by the FPIA (29).

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