Transcript labtox.PRE

Lab Medicine Conference :
Toxicology Studies
Jim Holliman, M.D., F.A.C.E.P.
Professor of Surgery and Emergency Medicine
Director, Center for International Emergency Medicine
M. S. Hershey Medical Center
Penn State University
Hershey, Pennsylvania, U.S.A.
Lab Medicine : Toxicology
Lecture Topics
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General use of toxicolgy studies
Toxicology lab methodologies
Components of drug & toxin screens
Clinical interpretation of drug & toxin
quantitative levels
ƒ Relative costs of different lab
methods
Drug Screens :
Types or Levels
ƒ Drugs of abuse
–Rapid urine assay
ƒ Comprehensive
ƒ Stat
ƒ Volatiles
Drug Screens :
Role in Clinical Care
ƒ Rarely has significant impact on clinical
care
ƒ Should be confirmatory
–Most diagnoses made from history,
physical, (& in some cases) other lab
tests
ƒ Rx is often required before the
information is available
Drug Screens :
Indications
ƒ Comatose patient with many potential
diagnoses or etiologies
ƒ Part of final brain death verification
ƒ Academic manuscript data
ƒ Psychiatric evaluation to rule out
organic brain syndrome
Drug Screens :
Caveats
ƒ Many false positives
ƒ Cross-reactions between substances
with similar structures
–Most commonly : over-the-counter
sympathomimetics react like
amphetamines
ƒ Information about false positives is
available from the company or lab
Drug Screens :
Caveats (cont.)
ƒ Many false negatives
ƒ Many screens are simply not comprehensive &
may not screen for the drug that is suspected
ƒ Drug can be present but below detection limit
–Dilute urines may have concentration below
detection limit
ƒ Rapid acting toxins (e.g., TCA's) may present
before significant amounts are excreted into
urine
Drug Screens : Importance of
Communication with the Laboratory
ƒ Request form should list confirmed
ingestants
ƒ List patient's medication history
ƒ List other suspected toxins
ƒ Lab can then prioritize screening to
look for specific drugs first, & shorten
turnaround time
Toxicology Screens : Role of
the Poison Control Center
ƒ Interpretation is everything
ƒ Expert advice is readily available
ƒ Many information systems accessed
Toxicology Lab :
Quantitative Analysis
ƒ Often critical to patient management
–Need for antidote administration
–Need for extracorporeal elimination
procedures
–Serial measurements help anticipate
therapies and proper setting for care
ƒ Be careful about measurement units
when interpreting results
Toxicology Lab :
Methodology
ƒ Thin - layer chromatography (TLC)
–e.g., Toxi-Lab
–Glass plate coated with absorbent (silicic
acid or aluminum oxide)
–Solvent system (varies by lab) sits at
base
–Specimen (usually urine) is added
–Interaction of drugs with sorbent &
solvent leads to partitioning
–Plate is dried & developed
Toxicology Lab :
Methodology for TLC (cont.)
ƒ Patterns are characteristic of toxins,
drugs, & their metabolites
ƒ Requires pattern recognition by
technician
–Experience = better results
ƒ Time consuming (typically 4 hours)
ƒ Cost : generally low for materials &
labor
Toxicology Lab Methodology :
Gas Chromatography (GC)
ƒ A volatile substance is dissolved in a solvent
(e.g., n-propanol)
ƒ Specimen injected into chromatograph & then
subjected to heat vaporization
ƒ Specimen is carried thru a column packed with
a substance (Carbowax or adiponitrile) that
alters the retention time
ƒ A detector (flame ionization, electron capture, or
nitrogen phosphorus) plots the emergence
Toxicology Lab Methodology :
Gas Chromatography (cont.)
ƒ Standard patterns compared for recognition of
substance
ƒ Area of the plot also correlates with quantification
ƒ Use of GC : is primary method for toxic alcohols
(methanol, ethylene glycol, isopropanol)
ƒ Problems :
–Set-up time & standardization required
–Not readily available at all hospital labs
–Expensive
Toxicology Lab Methodology : High
Pressure Liquid Chromatography (HPLC)
ƒ Methods & principles same as for GC,
but not restricted to volatiles
ƒ High pressure pump (1000 to 6000 psi)
facilitates movement thru the column
ƒ Accurate & reliable
ƒ Often used for comprehensive screens
ƒ Expensive
Tox Lab Methodology : Gas Chromatography Mass Spectrophotometry (GC - MS)
ƒ Same as GC, but then linked to a mass
spectrophotometer as emerges from
column
ƒ Like getting a fingerprint of a toxin
ƒ This is the "gold standard" for drug
screening
ƒ Is the confirmatory test for legal
specimens
Toxicology Lab Methodology : Enzyme Mediated Immunoassay Techniques (EMIT)
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Most commonly employed in hospitals
Rapid screening test
Easy to perform
Intermediate cost
However inaccuracies are common
Toxicology Lab Methodology :
EMIT Immunoassay System
ƒ Specimen usually urine ; may be serum
ƒ First, add antibody directed against the drug being
analyzed for
ƒ Then add the drug linked to an enzyme
ƒ When active, the enzyme converts NAD to NADH
ƒ NADH absorbs 355 nm wavelength light
ƒ Absorbance amount correlates with concentration
ƒ Spectrophotometer then records & quantifies
Toxicology Lab Methodology :
EMIT (cont.)
ƒ If no drug is in specimen, the antibody will bind
the drug - linked enzyme & inactivate this
reaction
ƒ If drug is present, it will bind the antibody,
leaving some drug - enzyme complexes active to
convert the NAD to NADH
ƒ Note : if low concentrations of drug are present,
only a small amount of NADH is produced ; this
yields result below detection cut-off & will be
reported as negative
Toxicology Lab Methodology :
Radioimmunoassay (RIA)
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Similar operation theory to EMIT
Quantity of drug-specific antibody added to specimen
Then known amount of radioactive-labeled drug added
Precipitate then analyzed with gamma counter
Emittance is inversely correlated with drug
concentration
ƒ Method is slow & expensive
ƒ Good for low concentration drugs (Digoxin, LSD,
paraquat)
Toxicology Lab Methodology :
Atomic Absorption Spectrophotometry
ƒ Most accurate & usual method for
inorganic agents
ƒ Used for metals :
–Iron
–Lead
–Arsenic
–Mercury
–Thallium
–Cadmium
Toxicology Lab :
H.M.C. Drugs of Abuse Screen
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Barbiturates
Cocaine
Amphetamines
Marijuana
Benzodiazepines
Opiates
At other labs, PCP is often added
Toxicology Lab : Drug Screens
ƒ Rapid turnaround, comprehensive, & stat tests
(by American Medical Labs) by class :
–Narcotics
–Amphetamines
–Phenothiazines
–Tranquilizers
–Hypnotics
–Antidepressants
–and 60 other drugs
Toxicology Lab : Drug Screens
Turnaround Time & Costs
Test
Specimens
Turnaround
Cost
Drugs of
Abuse
50 ml urine
2 hours
Rapid
Turnaround
50 ml urine or
14 ml blood
(2 red tops)
50 ml urine &
14 ml blood
16 to 40
hours
$ 100
($ 86 to
confirm)
$ 100 to
$ 260
Stat
(1 red, 1 grey)
4 to 10 hours
$ 440
Toxicology Lab :
Heavy Metal Screens
ƒ Includes : mercury, arsenic, lead
–Also available : thallium, cadmium
–Others may be available on request
ƒ Specimen is 24 hour urine collection in
special metal-free container
ƒ Spot urine can be used in emergency
situations
ƒ Symptoms & clinical presentation guide
therapy, since turnaround times are long
Toxicology Lab : Analysis for
Drugs or Toxins in Substances
ƒ Analysis for drugs or toxins (liquid or
solid) contained in other substances
can be performed by some labs
ƒ Used for checking for adulterants in
food products or health store herbal
medicines
Toxicology Lab Quantifications :
Ethanol
ƒ Gas chromatography is the "gold standard"
ƒ Enzymatic oxidative - reductive tests include
the alcohol dehydrogenase technique
–Interfering substances include methanol,
isopropanol, formaldehyde, & fluoride
ƒ Breathalyzers give result based upon blood
ethanol to breath ethanol ratio of 2100 : 1
ƒ Plasma or serum levels are 16 to 18 % higher
than whole blood levels
Toxicology Lab :
Ethanol (cont.)
ƒ Comparison of different unit
equivalents reported for legal
intoxication level :
–0.10 % weight / volume
–100 mg / deciliter or 100 mg %
–1.0 gram / liter
Toxicology Lab Quantifications :
Methanol
ƒ Assay : GC ; done on blood
ƒ Caveats : metabolized to formate ; late presenters
may not have detectable levels
ƒ Toxicity : serum bicarbonate & / or formate level
(by reference lab) provide best correlation
ƒ Interventions : Begin ethanol infusion if strong
history or osmolal gap evident while awaiting level
; for levels > 20 to 50 mg/dl, start ethanol infusion,
hemodialysis, & 4-methylpyrazole
Toxicology Lab Quantifications :
Isopropanol
ƒ Assay : GC or GLC ; done on serum
ƒ Caveat : metabolized to acetone ;
persistent acetone levels contribute to
prolonged CNS effects
ƒ Toxicity : > 50 mg / dl ; is twice as potent
as ethanol
ƒ Interventions : supportive care ; rarely
need hemodialysis
Toxicology Lab Quantifications :
Ethylene Glycol
ƒ Assay : GC with OV-1 column or mass spectrometry ; done
on serum
ƒ Caveat : late presenting cases (after metabolism to
glycolate & oxalate) may not have detectable levels
–Glycolate levels also available at reference labs
ƒ Toxicity : serum bicarbonate level may be better correlation
(glycolate level may also be better correlate)
ƒ Interventions : consider ethanol infusion when toxicity is
suspected while awaiting level
–For levels > 20 to 50 mg/dl : use ethanol infusion, also
hemodialysis, and consider 4-methylpyrazole
Toxicology Lab Quantifications :
Acetaminophen
ƒ Assay techniques : EMIT, GC, HPLC, Fluorescence
polarization immunoassay ; done on serum
–Colorimetric & other immunoassays may be
unrelable
ƒ Therapeutic level : 5 to 20 micrograms / ml
ƒ Toxicity : plot on Rumack - Matthew nomogram
–Based on 150 mcg/ml toxic at 4 hours & then a 4
hour half life
ƒ Antidote : n-acetylcysteine
–Begin Rx while awaiting level if > 8 hours from ingestion
Toxicology Lab Quantifications :
Salicylates
ƒ Assay : Old method is colorimetric (Trinder) ; now use
GC, HPLC, Fluorescence polarization ; done on serum
ƒ Caveat : chronic ingestions may have serious toxicity at
levels > 40 mg/dl
ƒ Therapeutic levels : 10 to 20 mg/dl
ƒ Toxicity : is time - dependent ; > 100 mg/dl is life
threatening ; clinical symptoms are best measure &
more useful than Done nomogram
ƒ Interventions : activated charcoal, alkaline diuresis,
hemodialysis if life - threatening
Toxicology Lab Quantifications :
Theophylline
ƒ Assay : EMIT, HPLC ("gold standard") ; done on
serum
ƒ Caveat : distinguish chronic from acute
ingestions
ƒ Therapeutic levels : 10 to 20 mcg/ml
ƒ Toxicity : potentially life - threatening at : > 100
mcg/ml if acute, > 40 mcg/ml if chronic
ƒ Interventions : activated charcoal, but is difficult
to control emesis ; hemoperfusion or dialysis
needed for life - threatening toxicity
Toxicology Lab Quantifications :
Phenytoin
ƒ Assay : GLC, HPLC, EMIT, RIA ; done on serum
ƒ Caveats : For therapeutic drug monitoring, the free
drug level may be most useful
ƒ Therapeutic levels : 10 to 20 mcg/ml
ƒ Toxicity : cardiovascular toxicity when given
intravenously is actually due to the diluent
propylene glycol ; acute oral poisoning does not
require cardiac monitoring
ƒ Interventions : supportive care & activated
charcoal
Toxicology Lab Quantifications :
Phenobarbital
ƒ Assay : EMIT, GC, RIA ; done on serum
ƒ Caveats : tolerance & enhanced enzyme
metabolism affect toxicity & its duration
ƒ Therapeutic levels : 15 to 40 mcg/ml
ƒ Toxicity : occurs if non-tolerant at 40 to
60 mcg/ml
ƒ Interventions : ventilatory &
cardiovascular support, multiple doses
activated charcoal, alkaline diuresis, +/hemodialysis
Toxicology Lab Quantifications :
Primidone (Mysoline)
ƒ Assay : GLC ; done on serum
ƒ Caveats : also metabolized to
phenobarbital
ƒ Therapeutic level : 5 to 15 mcg/ml, with 10
to 20 mcg/ml phenobarbital
ƒ Toxicity : same problems as for
phenobarbital
ƒ Interventions : supportive care
Toxicology Lab Quantifications :
Valproate (Depakote, etc.)
ƒ Assay : GC-MS, GC, GLC, EMIT ; done on serum
ƒ Caveats : distinguish between immediate & delayed
release products
ƒ Therapeutic levels : 50 to 100 mcg/ml
ƒ Toxicity : Beyond 100 mcg/ml increasing levels
correlate with worsening CNS depression
ƒ Interventions : activated charcoal ; anecdotal
reports of benefits from extracorporeal removal
Toxicology Lab Quantifications :
Carbamazepine
ƒ Assay : EMIT, HPLC ; done on serum
ƒ Caveat : active metabolite
carbamazepine epoxide not included as
part of results in EMIT assay
ƒ Therapeutic levels : 4 to 12 mcg/ml
ƒ Toxicity : good correlation with level ;
clinically apparent when > 20 mcg/ml
ƒ Interventions : activated charcoal ; rarely
need to consider hemoperfusion
Toxicology Lab Quantifications :
Digoxin
ƒ Assay : RIA, EMIT, Fluorescence polarization assay ; done
on serum
ƒ Caveat : spurious results with digoxin - like
immunoreactive substances (DLIS) or after digoxin Fab
(antibody fragment ) Rx
ƒ Therapeutic levels : 0.8 to 2.0 ng/ml
ƒ Toxicity : good correlation with chronic toxicity or with
acute toxicity after 6 hours from overdose (after the slow
alpha distribution phase)
ƒ Interventions : use digoxin antibody fragments (do not
wait for level if dysrhythmias are present)
Toxicology Lab Quantifications :
Procainamide
ƒ Assay : EMIT, HPLC, Fluorometric ; done on
serum
ƒ Caveats : metabolite N-acetylprocainamide
(NAPA) contributes to toxicity, especially in
renal insufficiency
ƒ Therapeutic levels : < 16 mcg/ml procainamide
–5 to 25 mcg/ml total procainamide + NAPA
ƒ Interventions : activated charcoal &
hemodialysis
Toxicology Lab Quantifications :
Quinidine
ƒ Asay : Fluorometric, EMIT, HPLC, GC-MS ; done on
serum
ƒ Caveat : quinine interferes with HPLC method
ƒ Therapeutic levels : 2 to 6 mcg/ml
ƒ Toxicity : usually seen at > 8 mcg/ml ; EKG and QRS
widening demonstrates toxicity
ƒ Interventions : consider serum alkalinization with
sodium bicarbonate +/- induced respiratory
alkalosis if intubated (acidified urine would
enhance elimination, but may aggravate toxicity)
Toxicology Lab Quantifications :
Lithium
ƒ Assay : spectrophotometric or emission flame photometry ;
done on serum (reported as meq/liter)
ƒ Caveats : Has very slow distribution phase, & so levels do
not always correlate with acute toxicity ; levels correlate
with neuromuscular symptoms
ƒ Therapeutic levels : 0.5 to 1.2 meq/L
ƒ Toxicity : acute when sustained > 3.0 meq/L ; chronic
toxicity may persist despite undetectable level
ƒ Interventions : IV fluid replacement, hemodialysis if acute >
4.0 meq/L, consider if chronic > 1.5 to 2.0 meq/L
Toxicology Lab Quantifications :
Iron
ƒ Assays : Atomic absorption spectrometry (AAS) is "gold
standard" ; colorimetric & RIA also available ; done on serum
ƒ Caveats : After deferoxamine, AAS is only reliable assay for
free iron
ƒ Toxicity : > 300 to 500 mcg/dl six hours after ingestion ; 500 to
1000 mcg/dl is serious, while > 1000 mcg/dl is potentially
lethal ; symptoms correlate well with levels
ƒ Interventions : chelation with deferoxamine ; chelation should
begin before level is returned in patients with cardiovascular
or CNS symptoms
ƒ Total iron binding capacity (TIBC) is no longer considered a
useful test
Toxicology Lab Quantifications :
Heavy Metals : Lead
ƒ Assay : whole blood lead is best by AAS ; free
erythrocyte protoporphyrin (FEP) is a good
screening test for severe lead exposure (> 60
mcg/dl)
ƒ Toxicity : see effects at > 10 mcg/d
ƒ Interventions : should do chelation if child level
> 25 mcg/dl, if adult > 25 to 40 mcg/dl
–Monitor & consider chelation with DMSA at >
15 mcg/dl ; if encephalopathy, use BAL &
EDTA, & consultation recommended
Toxicology Lab Quantifications :
Heavy Metals : Arsenic
ƒ Assay : 24 hour urine best ; also blood level by AAS
ƒ Caveats : check recent diet, as a seafood or kelp meal
will dramatically increase level
ƒ Toxicity : > 100mcg/Liter after acute exposure, but
toxicity may be present after chronic or severe acute
exposure with lower levels
ƒ Interventions : Chelators British Anti-Lewisite (BAL)
parenterally & oral dimercaptosuccinic acid (DMSA) or
D-penicillamine
–Consultation recommended
–Repeat assays monitor response to chelation
Toxicology Lab Quantifications :
Heavy Metals : Mercury
ƒ Assay : whole blood (especially inorganic
& elemental) by AAS or also on 24 hour
urine
ƒ Toxicity : blood level > 1 to 3 mcg/dl,
urine level > 50 mcg/Liter
ƒ Interventions : chelation with BAL, DMSA,
+/- penicillamine ; hemodialysis often
required
–Consultation recommended
Toxicology Lab Quantifications :
Paraquat
ƒ Assay : RIA ; done on serum ;
–Urine screen : Add 1 ml 1% sodium
dithiorite in 2N sodium hydroxide to 10
ml urine
blue color implies paraquat,
blue-green color implies diquat
ƒ Toxicity : Level & time post-ingestion
correlate with probability of survival
ƒ Interventions : hemoperfusion if < 8
hours from ingestion
Toxicology Lab Quantifications :
RBC & Plasma Cholinesterase Levels
ƒ Assays : Michel, Ellman, Delta pH, & micro-Michel methods
used
–Specimen is 10 ml heparinized tube & must separate plasma
within 1 to 2 hours
ƒ Caveats : Not useful in acute management ; clinical symptoms &
findings dictate therapy
ƒ Monitoring serum levels : useful technique to assess for
occupational exposure & return to work potential
–> 50 % reduction causes mild symptoms
–80 to 90 % reduction causes moderate symptoms
–> 90 % reduction causes severe symptoms
ƒ Interventions : Surface removal, decontamination, atropine,
pralidoxime, & supportive care
Lab Medicine : Toxicology
Summary
ƒ Should familiarize yourself with your lab's
toxicology assay capabilities
ƒ Consider lab results in light of clinical findings
–If lab result does not fit with clinical situation,
should confirm lab result with another test
ƒ Lab tests will sometimes be needed for afterthe-fact confirmation, even though they will not
affect acute clinical management