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Test Code FLUC 5-Flucytosine, Serum

Performing Laboratory

Mayo Medical Laboratories in Rochester

Reporting Name

5-Flucytosine, S

Specimen Type

Serum


Specimen Required


Container/Tube:

Preferred: Red top

Acceptable: Serum gel

Specimen Volume: 0.5 mL

Collection Instructions: 

1. Serum for a peak level should be drawn 1 to 2 hours after oral dose or 30 minutes after intravenous infusion. Trough specimens should be drawn immediately prior to next scheduled dose.

2. Spin down within 2 hours of draw.


Reject Due To

Hemolysis

Mild OK; Gross OK

Lipemia

Mild OK; Gross OK

Icterus

Mild OK; Gross OK

Other

Whole blood

Specimen Stability Information

Specimen Type Temperature Time
Serum Frozen (preferred) 28 days
  Ambient  28 days
  Refrigerated  28 days

Specimen Minimum Volume

0.3 mL

Day(s) and Time(s) Performed

Tuesday, Thursday, 9 a.m.

Specimen Retention Time

14 days

Analytic Time

2 days

Reference Values

Therapeutic concentration:

Peak >25.0 mcg/mL (difficult infections may require higher concentrations)

Toxic concentration:

Peak >100.0 mcg/mL

Useful For

Monitoring serum concentration during therapy

 

Evaluating potential toxicity

 

May be useful to evaluate patient compliance

Method Name

Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)

Test Classification

This test was developed and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.

CPT Code Information

80299

LOINC Code Information

Test ID Test Order Name Order LOINC Value
FLUC 5-Flucytosine, S In Process

 

Result ID Test Result Name Result LOINC Value
82741 5-Flucytosine, S 3639-2

Clinical Information

Flucytosine is a broad-spectrum antifungal agent generally used in combined therapy (often with amphotericin B) for treatment of fungal infections such as cryptococcal meningitis. Concerns with toxicity (bone marrow suppression, hepatic dysfunction) and development of fungal resistance limit the use of flucytosine, particularly as a monotherapy. The drug is well-absorbed orally, but can also be administered intravenously (available outside of the United States).

 

There is good correlation between serum concentrations of flucytosine with both efficacy and risk for toxicity. Because of the drugs short half-life (3-6 hours), therapeutic monitoring is typically performed at peak levels, 1 to 2 hours after an oral dose or 30 minutes after an intravenous administration.

 

Flucytosine is eliminated primarily as unmetabolized drug in urine. Patients with renal dysfunction may require dose adjustments or more frequent monitoring to ensure that serum concentrations do not accumulate to excessive levels. Nephrotoxicity associated with use of amphotericin B can affect elimination of flucytosine when the drugs are coadministered.

Interpretation

Most individuals display optimal response to flucytosine when peak serum levels (1-2 hours after oral dosing) are greater than 25.0 mcg/mL. Some infections may require higher concentrations for efficacy. Toxicity is more likely when peak serum concentrations are greater than 100.0 mcg/mL

Cautions

This test cannot be performed on whole blood. Serum must be separated from cells within 2 hours of draw.

Clinical Reference

1. Goodwin ML, Drew RH: Antifungal serum concentration monitoring: an update. J Antimicrob Chemother 2008;61:17-25 

2. Andes D, Pascual A, Marchetti O: Antifungal therapeutic drug monitoring: established and emerging indications. Antimicrob Agents Chemother 2009;53(1):24-34

Method Description

5-Flucytosine is extracted by mixing serum samples with acetonitrile and methanol mixture to precipitate proteins. The supernatant is removed and analyzed by an in-house developed liquid chromatography-tandem mass spectrometry (LC-MS/MS) method.(Unpublished Mayo method)