NOAA Technical Memorandum NMFS-NWFSC-122

ELISA Methods 

for Domoic Acid Quantification 

in Multiple Marine Mammal Species 

and Sample Matrices

May 2013

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service

Northwest Fisheries Science Center
NOAA Technical Memorandum

NMFS-NWFSC Series

The Northwest Fisheries Science Center of NOAA’s

National Marine Fisheries Service uses the NOAA Technical 

Memorandum NMFS-NWFSC series to issue scientific and 

technical publications. Manuscripts have been peer reviewed 

and edited. Documents published in this series can be cited 

in the scientific and technical literature.

The Northwest Fisheries Science Center’s NOAA Technical 

Memorandum NMFS-NWFSC series continues the NMFSF/NWC series established in 1970 by the Northwest and

Alaska Fisheries Science Center, which subsequently was 

divided into the Northwest Fisheries Science Center and the 

Alaska Fisheries Science Center. The latter center now uses 

the NOAA Technical Memorandum NMFS-AFSC series.

Reference throughout this document to trade names does not 

imply endorsement by the National Marine Fisheries Service.

This document should be referenced as follows:

Frame, E., and K. Lefebvre. 2013. ELISA methods for 

domoic acid quantification in multiple marine mammal 

species and sample matrices. U.S. Dept. Commer., NOAA 

Tech. Memo. NMFS-NWFSC-122, 20 p.
NOAA Technical Memorandum NMFS-NWFSC-122

ELISA Methods 

for Domoic Acid Quantification 

in Multiple Marine Mammal Species 

and Sample Matrices

Elizabeth Frame and Kathi Lefebvre

Northwest Fisheries Science Center

2725 Montlake Boulevard East

Seattle, Washington 98112

May 2013

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service
Most NOAA Technical Memorandums

NMFS-NWFSC are available at the 

Northwest Fisheries Science Center

Web site, http://www.nwfsc.noaa.gov

Copies are also available from the

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Phone orders: 1-800-553-6847

E-mail orders: orders@ntis.fedworld.gov

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Table of Contents

List of Figures...............................................................................................................................................v

List of Tables..............................................................................................................................................vii

Executive Summary.....................................................................................................................................ix

Acknowledgments........................................................................................................................................xi

Introduction...................................................................................................................................................1

Methods ........................................................................................................................................................3

Sample Collection.....................................................................................................................................3

California Sea Lions .............................................................................................................................3

Common Dolphins................................................................................................................................3

Northern Sea Otters...............................................................................................................................3

Ice Seals................................................................................................................................................3

Other Cetacean and Pinniped Species...................................................................................................3

Sample Handling.......................................................................................................................................3

Sample Extraction.....................................................................................................................................4

Fecal, Stomach Content, and Intestinal Content Samples.....................................................................4

Urine, Blood Serum, Milk, Bile, and Aqueous Humor Samples..........................................................4

DA Detection/quantification by Biosense ELISA ....................................................................................4

Matrix Effects Testing ..........................................................................................................................4

Measurement of DA..............................................................................................................................5

Quantification of DA ............................................................................................................................5

DA Detection/quantification by LC/MS/MS ............................................................................................5

Results...........................................................................................................................................................7

Biosense ELISA........................................................................................................................................7

LC/MS/MS................................................................................................................................................7

Discussion...................................................................................................................................................11

Conclusions.................................................................................................................................................13

References...................................................................................................................................................15

Appendix A: Domoic Acid Sample Collection and Storage Protocol ........................................................17

Appendix B: Domoic Acid Extraction Protocols........................................................................................19
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v

List of Figures

Figure 1. Matrix curves for Biosense ELISA from California sea lion feces, urine, serum, bile, 

milk, and stomach contents...........................................................................................................................8

Figure 2. Fecal matrix effects curves from Biosense ELISA for several marine mammal species.............8

Figure 3. Comparison of DA as measured by Biosense ELISA and LC/MS/MS for selected 

California sea lion fecal, urine, and serum samples......................................................................................9
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vii

List of Tables

Table 1. Comparison of DA values measured with the ELISA method and LC/MS/MS in California 

sea lion feces, urine, and serum ..................................................................................................................10

Table 2. Minimum dilution of sample extracts and minimum detection limits in sample material for 

selected species and sample types...............................................................................................................11

Table A-1. DA concentration in two sample matrices collected simultaneously from the same animal...17
viii
ix

Executive Summary

Over the past several years, considerable effort has been invested in developing sensitive 

methods for detecting algal toxins, specifically using enzyme-linked immunosorbent assay 

(ELISA) technologies. Previously, detection of algal toxins, such as domoic acid (DA), required 

expensive analytical equipment and highly trained personnel to perform high-performance liquid 

chromatography (HPLC) or liquid chromatography tandem mass spectrometry (LC/MS/MS) 

methodologies. While excellent methods exist for DA detection using these technologies for 

seafood safety programs, these methods lacked the sensitivity needed for researchers 

investigating the impacts of harmful algal bloom toxins in wildlife. The advent of ELISA kits 

for DA quantification provides a useful tool for diagnostic studies in natural marine mammal 

populations, but requires careful testing in new sample matrices before being employed. This 

technical memorandum provides protocols for DA quantification by ELISA in multiple marine 

mammal species and sample matrices and outlines consistent reporting criteria for use by the 

larger scientific community.
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xi

Acknowledgments

The authors especially thank Frances Gulland of The Marine Mammal Center for her 

guidance and mentorship through all of our work. We also thank the many folks involved in 

sample collection, storage, and delivery—which requires considerable effort without which this 

project would not have been possible—including Liz Wheeler, The Marine Mammal Center; 

Kathy Burek Huntington, Alaska Veterinary Pathology Services; members of the Alaska Marine 

Mammal Stranding Network; Verena Gill, U.S. Fish and Wildlife; Lori Quakenbush, Alaska 

Department of Fish and Game; and Kerri Danil, Southwest Fisheries Science Center. We 

appreciate the thoughtful comments and critiques of this manuscript from reviewers including 

Northwest Fisheries Science Center reviewers Nick Adams, Keri Baugh, Walt Dickhoff, and

Michelle McClure.
xii
Introduction

The problem of domoic acid (DA) poisoning in marine mammals has been well 

documented and tracked since it was first reported in the late 1990s in California sea lions 

(Zolophus californianus) from the central California coast (Lefebvre et al. 1999, Scholin et al. 

2000, Gulland et al. 2002). Cases of DA poisoning as large outbreaks, clusters of several 

animals, or isolated cases have consistently been reported since that time (Bejarano et al. 2008). 

Primary clinical signs of DA neuroexcitotoxicity consist of seizures and are regularly used for 

preliminary diagnosis, often with follow-up by histopathological examination for characteristic 

brain lesions and measurements of toxin content in various matrices such as feces, urine, and 

blood serum. DA detection and quantification previously relied heavily on high-performance 

liquid chromatography (HPLC) or liquid chromatography tandem mass spectrometry 

(LC/MS/MS), which both require specialized equipment or expertise to perform (Quilliam et al. 

1989, 1995, Hess et al. 2001, Tor et al. 2003, Leandro et al. 2010). Now, with the advent of 

commercially available enzyme-linked immunosorbent assay (ELISA)–based kits for DA 

quantification, any lab equipped with a plate reader has the capacity to detect and quantify DA.

Commercially available ELISA kits are excellent tools that allow for quick, convenient 

measurements of algal toxins in a large number of samples. They were originally developed for 

use with shellfish tissue; the details of sample preparation and dilution are well described in the 

product literature and instructional booklets for this particular matrix. However, these kits have 

not been thoroughly validated by the manufacturers for use with marine mammal sample 

matrices. Unlike shellfish samples, marine mammal samples come in many different matrix 

types, including feces, urine, serum, stomach contents, bile, and other bodily fluids. It should not 

come as any surprise that a fecal sample and a urine or serum sample are quite different in terms 

of matrix effects. “Matrix effects” refers to the interaction of the sample material itself with the 

ELISA reagents and the potential for components other than the target DA to result in a change 

in the absorbance signal, which will be interpreted as an incorrect DA concentration. For this 

reason, every new sample matrix needs to be tested to determine appropriate minimum dilutions 

to avoid these effects. Put simply, false positives can easily be obtained if samples are not 

adequately diluted.

Due to the issues related to matrix differences and the multiple techniques available for 

toxin detection, there is a need for a uniform protocol for processing and analyzing marine

mammal samples for DA quantification. These protocols must include validation for each type 

of matrix and a minimum dilution level requirement. When reporting DA values in wildlife 

samples, the method, minimum dilution, and sample quantification limit should be reported 

along with the DA values. Knowledge of the analytical technique, minimum dilution, and 

sample quantification limit is important because different laboratories may use different methods 

for DA measurement (e.g., ELISA, HPLC, LC/MS/MS) and the sensitivity of these methods 

varies depending on the type of analysis used and the sample matrix (see Table 4 in Lefebvre and 

Robertson 2010 for a comparison of commonly reported DA detection methods). This 
2

information must be made clear to those who are interpreting data from toxin analyses for 

wildlife health assessments. It should also be understood that the quantification limit of the 

instrument or kit is not the quantification limit for DA within the sample material itself (sample 

quantification limit). Sample quantification limits will usually be higher than that of the 

instrument or method due to dilutions needed for extraction, cleanup, and prevention of matrix 

effects.

The difference between sample and instrument quantification limits highlighted above 

explains why some samples can yield a positive result from ELISA, but not when analyzed by 

HPLC or LC/MS/MS. This can lead to confusion and uncertainty over results that are relayed to 

researchers, veterinarians, or other wildlife health professionals who receive the data. Further 

complications can arise when looking at past data analyzed via methods with higher minimum 

sample quantification limits. This is an increasing problem, as researchers are starting to report 

the “first” observation of DA in a region or species. Often these “firsts” are due to lack of 

previous sampling or the advent of more sensitive analytical methods.

The goal of this technical memorandum is to provide detailed extraction and 

quantification methods for DA in a variety of marine mammal samples from several 

representative species, including pinnipeds, cetaceans, and mustelids. Here we provide matrixspecific recommendations for the use of a commercially available ELISA kit for feces, blood, 

urine, bile, aqueous humor, and stomach contents. In addition to providing minimum dilutions 

for ELISA, a comparison of measurements of DA from selected species and matrix types using 

Biosense ELISA (Biosense Laboratories, Bergen, Norway) and LC/MS/MS is included.
3

Methods

Sample Collection

California Sea Lions

Samples of frozen California sea lion feces, urine, serum, bile, aqueous humor, stomach 

contents, and milk were provided from material at The Marine Mammal Center, Sausalito, 

California. Samples were collected from animals that stranded along the central California coast.

Common Dolphins

Samples of frozen dolphin (Delphinidae) feces, urine, and stomach contents were 

provided from material collected by the National Marine Fisheries Service’s Southwest Fisheries 

Science Center, La Jolla, California. These animals were collected from the waters off the 

southern California coast in the San Diego area.

Northern Sea Otters

Samples of frozen northern sea otter (Enhydra lutris) urine were obtained from animals 

collected by U.S. Fish and Wildlife agents in areas along the shores of the Gulf of Alaska. 

Frozen urine and blood samples were also provided by U.S. Geological Survey agents from 

animals found on coastal beaches of Washington State.

Ice Seals

Samples of frozen stomach and intestinal contents from subsistence-harvested ice seal 

species—ribbon (Histriophoca fasciata), ringed (Pusa hispida), spotted (Phoca largha), and 

bearded (Erignathus barbatus)—were collected from seven Alaskan villages along the shores of 

the Bering and Chukchi seas.

Other Cetacean and Pinniped Species

Fecal, stomach content, or urine samples from other species such as beluga whales

(Delphinapterus leucas), humpback whales (Megaptera novaeangliae), Steller sea lions 

(Eumetopias jubatus), harbor seals (Phoca vitulina), and northern fur seals (Callorhinus ursinus) 

were collected from stranded/dead animals by stranding network members from organizations in 

Alaska, Washington, and California.

Sample Handling

All samples were placed in plastic screw-cap tubes or Whirl-Pak baggies (Nasco, Salida, 

California) and frozen at or below −20°C as soon after collection as possible. Samples were kept 
4

frozen until shipped to the Northwest Fisheries Science Center for analysis. Frozen samples 

were shipped on dry ice overnight from the collecting agency and stored frozen at −20°C until 

analysis.

Sample Extraction

All samples were thawed at room temperature. Depending on the amount of sample 

available, 1 to 4 g of sample was weighed out into a 15 mL polypropylene screw-cap tube (BD 

Falcon, BD Biosciences, San Jose, California). The initial extraction step was carried out by 

adding 50% aqueous methanol to the sample in a 1 in 4 wt/wt ratio (1 part sample, 3 parts 50% 

MeOH) and thoroughly vortexing the sample.

Fecal, Stomach Content, and Intestinal Content Samples

Samples were homogenized for at least 60 seconds using an Omni ES homogenizer

(Omni International, Kennesaw, Georgia). The homogenized sample was then spun at 10,000 

× g in a Sorvall RC 5C Plus centrifuge (Thermo Fisher Scientific Inc., Waltham, Massachusetts) 

for 20 minutes at 4°C. The supernatant was then filtered through a 0.22 μm membrane 

microcentrifuge tube filter (EMD Millipore, Billerica, Massachusetts) and spun in a desktop 

microcentrifuge for 10 minutes at 12,000 × g. Samples were stored at 4°C until analysis by 

ELISA or LC/MS/MS.

Urine, Blood Serum, Milk, Bile, and Aqueous Humor Samples

Samples were sonicated with a Branson Sonifier 450 (Branson Ultrasonics Corp., 

Danbury, Connecticut) at 50% pulse for 45 seconds at a setting of 5. Samples were then 

centrifuged at 10,000 × g in a Sorvall for 20 minutes at 4°C. The supernatant was then filtered 

through a 25 mm diameter, 0.45 μm pore size syringe filter (Pall Corp., Port Washington, New 

York). Samples were stored at 4°C until analysis by ELISA or LC/MS/MS.

DA Detection/quantification by Biosense ELISA

Matrix Effects Testing

Matrix effects testing was performed using material from animals known to be negative 

for DA. A dilution curve was made of the methanol extracts at the following dilutions: 1:1, 1:5, 

1:10, 1:20, 1:40, 1:80, 1:160, 1:320, 1:640, 1:1280, 1:2560, and 1:5120. From this curve, the 

minimum dilution necessary to avoid matrix effects was determined for each species and sample 

matrix by plotting the absorbance (A) at each dilution with respect to the maximum absorbance 

possible (Amax). Ideally, A/Amax will approach a value of 1 at some dilution level. The slope of 

the curve is steeper at the lower end of the dilution series, but levels out as samples are diluted 

enough to overcome matrix effects. A value of 1 indicates that A (the absorbance of the DAnegative matrix control sample) is equal to that of Amax (the absorbance value in the DA-negative

control). The dilution which corresponds to this flattening of the curve of A/Amax vs. dilution is 

the minimum dilution needed to avoid matrix effects.
5

The methanol in the sample was not a contributing factor in terms of matrix effects. The 

sample dilution buffer used with the Biosense kit is 10% methanol in PBS-Tween (PBS-T). All 

sample dilutions listed above resulted in a methanol content less than 10%, with the exception of 

the 1:1 (undiluted) extract, which had a methanol content of 37.5%.

Measurement of DA

Biosense ELISA measurements of DA were performed as described in the instruction 

protocol supplied by the manufacturer. Samples were diluted with sample buffer (10% methanol 

in PBS-T) as described above. In addition to samples, each plate included a 10-point standard 

curve, an Amax control (maximum binding of anti-DA-HRP conjugate, yielding maximum 

absorbance) and an Amin control (background absorbance of TMB peroxidase substrate, no antiDA-HRP conjugate, yielding minimum absorbance). All samples, standards, Amax, and controls 

were run in duplicate. Precoated plates were soaked in washing buffer (PBS-T) for 10 minutes, 

then buffer was removed and sample was added to duplicate wells. Anti-DA-HRP conjugate 

was added to each well (except blank) and the plate was sealed and incubated for 1 hour in the 

dark at room temperature. Liquid was then removed and the plate washed four times with 

washing buffer (PBS-T) in a BioTek ELx50 plate washer (BioTek, Winooski, Vermont). TMB 

peroxidase substrate was added and the plate was incubated in the dark at room temperature for 

15 minutes. The reaction was stopped with 0.3 molar H2SO4 and absorbance at 450 nm was read 

after 2 minutes on a VERSAmax microplate reader (Molecular Devices LLC, Sunnyvale, 

California).

Quantification of DA

A four-parameter logistic curve fit model is used by the software included with the 

Biosense ELISA kit to generate a DA standard calibration curve. Based on a working range 

determined by the I20 and I80 values from the calibration curve, DA values in samples were 

tagged as “within range,” “too dilute,” or “too concentrated.” DA values from samples tagged 

“within range” were accepted. Values tagged as “too dilute” were given a value of “below 

quantification limit.” Samples tagged “too concentrated” were diluted and rerun until obtaining 

a “within range” value.

DA Detection/quantification by LC/MS/MS

A Waters Agilent Micromass Quatromicro triple quadrupole electrospray tandem mass 

spectrometer was used for all samples. Separation was carried out using an Acquity UPLC BEH 

C-18 1.7 μm particle size, 2.1 x 100 mm column (Waters Corp., Milford, Massachusetts). 

Solvent A was water with 50 mM formic acid and Solvent B was 95% acetonitrile with 50 mM

formic acid. The run consisted of a 3 minute gradient from 95% to 85% Solvent A, followed by 

a 0.5 minute hold at 85% Solvent A, followed by a 1.5 minute gradient return to 95% Solvent A. 

Injection volume was 20 μL and the flow rate was 0.8 mL/minute. Column oven temperature 

was 40°C. The retention time for DA was 1.4 minutes. LC/MS/MS data was obtained in the 

positive ionization electrospray ionization (ESI+) mode. The following settings were used: 

capillary voltage 2.6 kV, cone voltage 30 V, extractor voltage 8 V, and RF lens voltage 3 V. 

Source temperature was 125°C and desolvation temperature was 390°C. Confirmation was 

carried out with three ion products: m/z 266, m/z 248, and m/z 166.
6

Quantification was performed using the product ion of m/z 266. A standard curve was 

run for each set of samples. The lower detection limit, defined as 3 × background, was 5 ng/mL.
7

Results

Biosense ELISA

Matrix effects curves were performed for feces, urine, serum, bile, milk, stomach 

contents, and aqueous humor from DA-negative California sea lions (Figure 1). From these 

curves, minimum dilutions of 1:100 (feces), 1:10 (urine) 1:10 (serum), 1:100 (bile), 1:50 (milk), 

1:50 (stomach contents), and 1:10 (aqueous humor) for the methanol extracts were determined to 

be necessary in order to avoid matrix effects. With the original 1 in 4 dilution performed during 

the extraction step, this translates into a total dilution of 1:400 (feces), 1:40 (urine), 1:40 (serum), 

1:400 (bile), 1:200 (milk), 1:200 (stomach contents) and 1:40 (aqueous humor) in terms of the 

original sample material. The results from more than 90 ELISA assays, representing 3,240 

samples, have generated working ranges varying between a mean minimum DA concentration of 

9.8 (±2.9) pg/mL and a mean maximum DA concentration of 192 (±34.9) pg/mL. Based on 

these assays, we have established a standard minimum quantification limit for diluted extracts of 

10 pg/mL, revising upwards in rare cases when the minimum I20 for a calibration curve is above 

a value corresponding to 10 pg/mL. When minimum dilutions to avoid matrix effects are taken 

into account, this means a minimum quantification level of 4 ng/g (or mL) for feces and bile, 2 

ng/g (or mL) for milk and stomach contents, and 0.4 ng/g (or mL) for urine, serum, and aqueous 

humor in the original sample material.

Matrix effects for California sea lions were explored in the greatest number of sample 

matrices, due to access to large numbers of samples from which DA-negative material was 

available. Other species are not as well represented in our sampling and hence do not have as 

comprehensive matrix testing from such a wide range of sample types. Fecal material has been 

available for most other species and matrix effects tests have been run for common dolphins, 

beluga whales, humpback whales, Steller sea lions, bearded seals, ringed seals, ribbon seals, 

spotted seals, and northern sea otters (Figure 2). The curves vary with species, but level off in 

the extract dilution range of 1:100. There may be slight variation, but a minimum dilution of 

1:100 for fecal extracts is appropriate for all species.

LC/MS/MS

A large number of DA-positive samples were available from California sea lions and 

were used for a comparison of Biosense ELISA and LC/MS/MS methods in fecal, urine, and 

serum matrices. Measurements of DA concentration by the ELISA and LC/MS/MS methods 

were in strong agreement (Figure 3). The Pearson product moment correlation was 0.989 

(P < 0.001), indicating a tight correlation between DA quantification performed using the two 

measurement methods. The two methods were also in close agreement in terms of actual 

concentrations, with the slope of the linear regression being 0.858 (r2 = 0.981).
8

Figure 1. Matrix curves for Biosense ELISA from California sea lion feces, urine, serum, bile, milk, and 

stomach contents. Matrix effects are eliminated with the extract dilution indicated by the 

flattening curve.

Figure 2. Fecal matrix effects curves from Biosense ELISA for several marine mammal species.

Dilution of sample extract

1 10 100 1000

A/Amax

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 Feces

Urine

Serum

Bile

Milk

Stomach contents

Dilution of sample extract

1 10 100 1000

A/Amax

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 California sea lion

Common dolphin

Beluga whale

Humpback whale

Steller sea lion

Northern fur seal

Ringed seal

Spotted seal

Bearded seal

Northern sea otter
9

Figure 3. Comparison of DA as measured by Biosense ELISA and LC/MS/MS for selected California sea 

lion fecal, urine, and serum samples.

A DA standard at 5 ng/mL (5 ppb) was quantifiable (defined as >3 × background), while 

the standard at 1 ng/mL (1 ppb) was detectable, but not 3 × above background. The actual 

sample detection limits were higher in California sea lion sample material. Sample extracts were 

run undiluted for LC/MS/MS, but the extraction protocol itself involves a 1:4 dilution; so to be 

reliability quantifiable, the samples must have a DA concentration of approximately 20 ng/g in 

the original sample material, but the actual concentration needed for detection will be higher 

depending on the matrix effects. The quantification limit of the LC/MS/MS method (5 ng/g) is 

considerably higher than for the ELISA method (10 pg/g), which can lead to confusion due to 

positive ELISA results and negative LC/MS/MS findings for the same sample. Table 1 lists 

several examples of positive results on ELISA that are below the detection limit on LC/MS/MS; 

hence these would appear negative for DA if the LC/MS/MS method were used, but positive if 

the ELISA method were employed.

All (18) fecal samples with DA concentrations above 80 ng/g (as determined by Biosense 

ELISA) were confirmed by the LC/MS/MS method (Table 1). Three fecal samples ranging from 

20 to 37 ng/g (as determined by Biosense ELISA) were also confirmed by LC/MS/MS, but 51 

fecal samples with DA concentrations between 4 and 80 ng/g (as determined by Biosense 

ELISA) could not be accurately quantified by LC/MS/MS (>3 × background). This range 

reflects the high variability in matrix effects in fecal samples, which vary widely due to prey type 

and animal condition.

ELISA DA (ng/g)

100 101 102 103 104 105

LC/MS//MS DA (ng/g)

100

101

102

103

104

105
10

Detection by LC/MS/MS was more sensitive in the urine samples. Urine samples with 

DA concentrations above 29 ng/g (as determined by Biosense ELISA) were reliably quantifiable. 

Twenty-four samples with DA concentrations between 0.4 and 29 ng/g were not quantifiable by

LC/MS/MS. The matrix effect from urine is much lower than from feces, making the 

quantification limit for urine samples closer to the expected limit of approximately 28 ng/g based 

on detection of clean DA standards.

Only 2 serum samples out of 25 measured in the study were quantifiable by LC/MS/MS. 

One of these was the highest DA positive serum sample as measured by ELISA at 72.2 ng/g and 

the other had a DA concentration of 7.0 ng/g. Twenty-three other serum samples between 0.4 

and 7 ng/g were not quantifiable by LC/MS/MS.

LC/MS/MS performed on other species including sea otters and common dolphins has 

yielded similar agreement with ELISA results: 138.2 ng/g, 129.8 ng/g, and 34.7 ng/g for 

LC/MS/MS vs. 149.9 ng/g, 141.6 ng/g, and 42.5 ng/g, respectively in feces from common 

dolphins collected off San Diego, California.

Table 1. Comparison of DA values measured with the ELISA method and LC/MS/MS in California sea 

lion feces, urine, and serum. Asterisk (*) indicates not quantifiable (>3 × background).

DA ng/g feces DA ng/g urine DA ng/g serum

ELISA LC/MS/MS ELISA LC/MS/MS ELISA LC/MS/MS

45,800 35,486 2,446 2,607 72 46

36,386 36,864 296 659 7 13

9,560 6,245 254 447 2.7 *

6,342 5,665 153 180 1.8 *

2,067 1,925 135 278 — —

1,075 1,470 97 45 — —

899 1,118 88 134 — —

852 1,974 52 23 — —

648 1,196 33 68 — —

626 537 29 11 — —

415 533 26 * — —

226 * 24 * — —

175 74 18 * — —

151 174 — — — —

147 149 — — — —

123 * — — — —

108 * — — — —

81 14 — — — —

37 40 — — — —

23 12 — — — —

20 12 — — — —
11

Discussion

The ELISA method is becoming more and more widespread as a technique for measuring 

DA in natural samples, and as the use of prepackaged kits broadens, there should be consistency 

in reporting results obtained from these kits. This method is quick and efficient—allowing 

processing of multiple samples in a short period of time—but with this convenience comes the 

potential danger of employing the method without regard for issues such as matrix effects. This 

is important for sample types that were not the original intended target of these kits, such as 

marine mammal material. These samples require different treatment from the shellfish samples 

for which the kits were designed and validated by the manufacturer, especially when it comes to 

minimum sample dilution.

This report describes in detail the minimum dilutions needed for analysis of DA by 

Biosense ELISA for a wide range of California sea lion sample types and, to a lesser degree,

sample types in several other species. Based on these results, it is clear that the variation 

between samples matrices is greater than that between species, though some interspecies 

differences are apparent, especially in “messier” matrices such as stomach contents, likely 

reflecting differences in diet. For this reason, we recommend the following general dilutions for 

sample extracts when matrix curves are not yet available: feces 1:100, urine 1:10, serum 1:10, 

aqueous humor 1:10, stomach contents 1:50, milk 1:50, and bile 1:100 (Table 2).

It is also important to realize that the detection limit of the Biosense ELISA kit, cited in 

the kit directions as 2 pg/g, is not the detection limit for DA in marine mammal matrices. Based 

on more than 3,000 sample runs, we have assigned a “practical” quantification limit of the kit at

10 pg/g. Using this as the default quantification limit in the diluted sample extracts run on the 

Table 2. Minimum dilution of sample extracts and minimum detection limits in sample material for 

selected species and sample types. Asterisk (*) indicates matrix type not tested for this species.

Matrix type

Species Feces Urine

Stomach 

contents Serum

California sea lion 100/4 10/0.4 50/2 10/0.4

Harbor seal 100/4 10/0.4 25/1 *

Ice seal spp. 100/4 10/0.4 50/2 *

Northern fur seal 100/4 10/0.4 25/1 10/0.4

Common dolphin 100/4 10/0.4 50/2 *

Sea otter 100/4 10/0.4 25/1 10/0.4

Steller sea lion 100/4 10/0.4 50/2 *

Humpback whale 100/4 * 50/2 10/0.4

Beluga whale 100/4 10/0.4 50/2 *
12

ELISA plate, and the minimum dilutions cited, the quantification limits in the original sample 

material are much higher; for example, a fecal sample is diluted 1:4 during extraction, then an 

additional 1:100 to avoid matrix affects, yielding a total dilution of 1:400. A quantification limit 

of 10 pg/g for this diluted sample translates to a 4,000 pg/g or 4 ng/g quantification limit in the 

original sample material. This sample quantification limit is still over two orders of magnitude 

lower than quantification limits reported for HPLC analyses and over one order lower than that 

for LC/MS/MS, the more commonly used methods historically.

The differences in DA quantification limits between ELISA, LC/MS/MS, and HPLC

continue to be a source of confusion. The ELISA is the most sensitive method and, as its use 

increases, questions about meaningful toxin levels in relation to toxicological impacts in wildlife 

become more important. For example, many reports of “first” records of DA in a certain area or 

species as quantified by ELISA may not be firsts at all—especially when the only method 

previously applied in the area or species has been HPLC. It is important to report the previous 

detection method and the limits of detection of that method when declaring these first reports. It 

is also important to establish baseline levels of DA presence in each species of interest before 

attributing the detection of the toxin to disease, death, or any type of toxicological impact in 

wildlife populations.
13

Conclusions

The Biosense ELISA is a robust, quick, and efficient tool for measuring DA in marine 

mammal samples when the proper dilutions are used and reported with the results. Biosense 

ELISA results agree well with results from LC/MS/MS analysis and are far more sensitive and 

specific than from HPLC. By using the extraction and dilution guidelines outlined here, reports 

of DA in marine mammal sample material can be standardized and used for comparison purposes 

across temporal and spatial ranges. This will become increasingly important as the method gains 

users and changing environmental factors such as global climate change potentially contribute to 

expansion in range or duration of DA events.
14
15

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VanDolah, and T. K. Rowles. 2008. Demographics and spatio-temporal signature of the biotoxin 

domoic acid in California sea lion (Zalophus californianus) stranding records. Mar. Mamm. Sci. 

24(4):899–912.

Gulland, F. M. D., M. Haulena, D. Fauquier, G. Langlois, M. E. Lander, T. Zabka, and R. Duerr. 2002. 

Domoic acid toxicity in California sea lions (Zalophus californianus): Clinical signs, treatment,

and survival. Vet. Rec. 150(15):475–480.

Hess, P., S. Gallacher, L. A. Bates, N. Brown, and M. A. Quilliam. 2001. Determination and 

confirmation of the amnesic shellfish poisoning toxin, domoic acid, in shellfish from Scotland by 

liquid chromatography and mass spectrometry. J. AOAC Int. 84:1657–1667.

Leandro, L. F., R. M. Rolland, P. B. Roth, N. Lundholm, Z. Wang, and G. J. Doucette. 2010. Exposure 

of the North Atlantic right whale Eubalaena glacialis to the marine algal biotoxin, domoic acid. 

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Lefebvre, K. A., C. L., Powell, M. Busman, G. J. Doucette, P. D. R. Moeller, J. B. Silver, P. E. Miller, M.

P. Hughes, S. Singaram, M. W. Silver, and R. S. Tjeerdema. 1999. Detection of domoic acid in 

northern anchovies and California sea lions associated with an unusual mortality event. Nat. 

Toxins 7(3):85–92.

Lefebvre, K. A., and A. Robertson. 2010. Domoic acid and human exposure risks: A review. Toxicon 

56:218–230.

Quilliam, M. A., P. G. Sim, A. W. McCulloch, and A. G. McInnes. 1989. High-performance liquid 

chromatography of domoic acid, a marine neurotoxin, with application to shellfish and plankton.

Int. J. Environ. Anal. Chem. 36:139–154.

Quilliam, M. A., M. Xie, and W. R. Hardstaff. 1995. Rapid extraction and cleanup for liquidchromatographic determination of domoic acid in unsalted seafood. J. AOAC Int. 78:543–554.

Scholin, C. A., F. Gulland, G. J. Doucette, S. Benson, M. Busman, F. P. Chavez, J. Cordaro, R. DeLong, 

A. De Vogelaere, J. Harvery, M. Haulena, K. Lefebvre, T. Lipscomb, S. Loscutoff, L. J. 

Lowenstein, R. Marin, P. E. Miller, W. A. McLellan, P. D. R. Moeller, C. L. Powell, T. Rowles, 

P. Silvagni, M. Silver, T. Spraker, V. Trainer, and F. M. VanDolah. 2000. Mortality of sea lions 

along the central California coast linked to a toxic diatom bloom. Nature 403(6765):80–84.

Tor, E. R., B. Puschner, and W. E. Whitehead. 2003. Rapid determination of domoic acid in serum and 

urine by liquid chromatography-electrospray tandem mass spectrometry. J. Agric. Food Chem. 

51:1791–1796.
16
17

Appendix A: Domoic Acid Sample Collection and 

Storage Protocol

For detection of domoic acid (DA), the most reliable sample matrix (type) is feces (Table 

A-1). Urine, stomach contents, blood (serum), and other fluids (milk, amniotic, etc.) are also 

useful, but DA clears relatively quickly from blood, so serum samples must be taken from 

relatively recent strandings/deaths.

Collect material as soon as possible. Heat (above 40°C) and light degrade DA. Collect 

samples in plastic tubes or Whirl-Pak baggies (Nasco, Salida, California). If liquid 

chromatography tandem mass spectrometry analysis will be used, it is best to avoid any 

polystyrene products, (polyethylene is acceptable). At least 1 g (or mL for fluids) is needed for 

analysis, but 4 g (or mL) are recommended. Freeze samples as soon as possible and avoid 

thawing/refreezing.

Table A-1. DA concentration in two sample matrices (feces and urine) collected simultaneously from the 

same animal. Note the DA concentration in the fecal sample is almost always significantly higher 

than in the urine sample.

CSL no. DA ng/g feces DA ng/mL urine

6615 45,800.0 87.6

6858 9,560.0 135.0

6857 1,612.2 253.7

7181 648.0 17.5

6695 150.8 29.4

6921 43.0 5.8

6986 36.7 4.9

6887 20.9 0.7

7160 11.3 0.9

6836 10.1 1.1

6958 8.3 0.7

7157 5.9 0.6

7021 4.3 1.1

6958 3.9 0.7

7157 3.8 0.6

6884 3.6 0.6

6882 1.4 1.9

6874 1.2 0.3

6911 1.2 0.3

6727 1.0 0.1

7013 0.7 0.7

6704 0.7 1.0
18
19

Appendix B: Domoic Acid Extraction Protocols

Feces

1. Thaw sample.

2. Weigh out 4 g into a 50 mL plastic tube.

3. Dilute 1:4 with 50% methanol (4 g of feces, 12 mL of 50% methanol).

4. Homogenize using a tissue homogenizer until the sample is well ground—usually at least 60 

seconds (keep samples on ice, since the homogenization generates heat).

5. Transfer homogenized sample into a centrifuge tube.

6. Centrifuge at 10,000 × g for 20 minutes at 4°C.

7. Transfer supernatant to a glass or plastic tube.

8. Filter supernatant using a microcentrifuge spin column. (Note: syringe filters can be used, 

but the fecal samples are often quite thick and require much patience. A prefiltration step 

using GF/C filters is often helpful.)

9. Depending on the volume, samples can be stored in glass screw-top vials or plastic tubes.

10. Store sample at 4°C (refrigerator). Do not freeze. Samples will remain stable for weeks, but 

it is best to analyze them as soon after extraction as possible.

Urine

1. Thaw sample.

2. Transfer 2 mL to a 15 mL plastic tube.

3. Dilute 1:4 with 50% methanol (2 mL of urine, 6 mL of 50% methanol).

4. Sonicate at 50% for 45–60 seconds (keep samples on ice).

5. Transfer sonicated sample to a centrifuge tube.

6. Centrifuge at 10,000 × g for 20 minutes at 4°C.

7. Transfer supernatant to a glass or plastic tube.

8. Filter supernatant using a 0.45 μm syringe filter.

9. Depending on the volume, samples can be stored in glass screw-top vials or plastic tubes.

10. Store sample at 4°C (refrigerator). Do not freeze. Samples will remain stable for weeks, but 

it is best to analyze them as soon after extraction as possible.
20

Serum

Follow procedure for urine.

Stomach Contents

Follow procedure for feces.

Other Body Fluids (Milk, Amniotic Fluid, Peritoneal Fluid, Etc.)

Follow procedure for urine.
Recent NOAA Technical Memorandums

published by the

Northwest Fisheries Science Center

NOAA Technical Memorandum NMFS-NWFSC121 Lian, C.E. 2012. West Coast limited entry groundfish cost earnings survey: Protocol and results for 

2008. U.S. Dept. Commer., NOAA Tech. Memo. NMFS-NWFSC-121, 62 p. NTIS number PB2013-

104899.

120 Pollock, M.M., J.M. Wheaton, N. Bouwes, C. Volk, N. Weber, and C.E. Jordan. 2012. Working 

with beaver to restore salmon habitat in the Bridge Creek intensively monitored watershed: Design 

rationale and hypotheses. U.S. Dept. Commer., NOAA Tech. Memo. NMFS-NWFSC-120, 47 p. NTIS 

number PB2013-101722.

119 Waples, R.S., K. Hindar, and J.J. Hard. 2012. Genetic risks associated with marine aquaculture. 

U.S. Dept. Commer., NOAA Tech. Memo. NMFS-NWFSC-119, 149 p. NTIS number PB2013-101344.

118 Stout, H.A., P.W. Lawson, D.L. Bottom, T.D. Cooney, M.J. Ford, C.E. Jordan, R.G. Kope, 

L.M. Kruzic, G.R. Pess, G.H. Reeves, M.D. Scheuerell, T.C. Wainwright, R.S. Waples, E. Ward, 

L.A. Weitkamp, J.G. Williams, and T.H. Williams. 2012. Scientific conclusions of the status review 

for Oregon coast coho salmon (Oncorhynchus kisutch). U.S. Dept. Commer., NOAA Tech. Memo. 

NMFS-NWFSC-118, 242 p. NTIS number PB2012-113458.

117 Maynard, D.J., T.A. Flagg, W.C. McAuley, D.A. Frost, B. Kluver, M.R. Wastel, J.E. Colt, and 

W.W. Dickhoff. 2012. Fish culture technology and practices for captive broodstock rearing of ESAlisted salmon stocks. U.S. Dept. Commer., NOAA Tech. Memo. NMFS-NWFSC-117, 65 p. NTIS 

number PB2012-110667.

116 Lian, C.E. 2012. West Coast open access groundfish and salmon troller survey: Protocol and results 

for 2005 and 2006. U.S. Dept. Commer., NOAA Tech. Memo. NMFS-NWFSC-116, 52 p. NTIS 

number PB2012-107486.

115 Plummer, M.L., W. Morrison, and E. Steiner. 2012. Allocation of fishery harvests under the 

Magnuson-Stevens Fishery Conservation and Management Act: Principles and practice. U.S. Dept. 

Commer., NOAA Tech. Memo. NMFS-NWFSC-115, 84 p. NTIS number PB2012-107485.

114 Bradburn, M.J., A.A. Keller, and B.H. Horness. 2011. The 2003 to 2008 U.S. West Coast bottom 

trawl surveys of groundfish resources off Washington, Oregon, and California: Estimates of distribution, 

abundance, length, and age composition. U.S. Dept. Commer., NOAA Tech. Memo. NMFS-NWFSC114, 323 p. NTIS number PB2012-105507.

Most NOAA Technical Memorandums NMFS-NWFSC are available at the

Northwest Fisheries Science Center Web site, http://www.nwfsc.noaa.gov