Does Intravenous Fluorescein Interfere With Clinical Laboratory Testing?

Blood and urine laboratory samples taken subsequent to administration of intravenous fluorescein may demonstrate false abnormal values, depending upon the sensitivity of different laboratory instruments, or patient variability. Fluorescein angiography is so widely used as a diagnostic tool in ophthalmology that we often fail to pause and consider its effects on the diagnosis of other medical conditions that the patient may have. Fluorescein absorbs light between 465 and 490 nm and emits fluorescence at a wavelength of 520 to 530 nm. Fluorescence is also used throughout clinical medicine as a diagnostic marker in certain clinical pathology tests and many more clinical chemistry tests that use ultraviolet and visible light spectrophotometers to determine diagnostic assay values. The possibility therefore exists that intravenous fluorescein for angiography may interfere with subsequent evaluations. The patient is often questioned about potential allergic contraindications, but rarely about any upcoming laboratory studies. In addition, the physician often evaluates a patient with an inflammatory retinal lesion, performs fluorescein angiography, and then sends the patient for diagnostic laboratory testing. It is therefore important to consider how large an effect fluorescein might have on other laboratory tests and for how long this effect lasts after injection. The serum concentration of intravenous fluorescein reaches its maximum within one minute after injection. 1 It is then metabolized in the liver to fluorescein glucuronide which is less fluorescent. It is subsequently excreted by the kidneys in both the free and glucuronidated form. In patients with normal metabolism and excretion, essentially all of the intravenous fluorescein dose is excreted with 24 hours after injection. Certain laboratory evaluations, such as creatinine clearance, require the collection of a 24-hour specimen which could contain a significant amount of fluorescein if the collection were begun shortly after the injection. Furthermore, since fluorescein binds to albumin, a major blood protein, there is the potential of displacing other substances that bind to albumin such as salicylates (aspirin), thereby increasing the free serum concentration without changing the total serum concentration. This could possibly alter assays of certain therapeutic drug concentrations which could affect the dose which the patient receives. In addition, the fluorescein is present in a concentration of approximately 1 microgram/ml soon after injection) whereas laboratory tests which use fluorescence as the diagnostic chromophore detect nanogram concentrations of fluorescent molecules. The concentration measured in laboratory tests can therefore be 0.1% of the peak serum concentions. The problem of interference can be avoided entirely by waiting until all the fluorescein has been excreted, but because of medical necessity or inattention to the possibility of interference, this may not be done. Method-dependent Results The interference of fluorescein angiography with diagnostic testing was first recognized in 1969 by Pinkerton in a patient being evaluated for hypertension and circulating catecholamines. 2 Other reports have focused on serum magnesium, protein, and digoxin. , A comprehensive study of analyses was performed by Bloom et al in which 5-ml of 10% fluorescein sodium was intravenously administered to 4 individuals and blood and urine collected for analysis at various time points. 5 In addition, this study utilized multiple techniques of analysis for some tests and demonstrated that the method of analysis as well as the substance tested must be considered. The great majority of common laboratory tests are not affected by the presence of intravenous fluorescein (Table 1). In the study by Bloom utilizing a Beckman CX3 instrument which uses a colorimetric endpoint, analysis of 24hour urine collections were not sensitive to the presence of fluorescein, in spite of the presence of the entire dose of fluorescein within each sample. On the other hand, under identical circumstances, the Abbott TDx (a common assay instrument which relies on fluorescence polarization) will show a great deal of interference. Single time point urine testing was not done, as most quantitative urine testing is performed on 24-hour collections. Blood glucose measurements, done either in the clinical pathology laboratory or with the current technology of home analyzers,6 are not sensitive to the presence of intravenous fluorescein. The most common serum or plasma tests which have been reported to be sensitive to the presence of fluorescein are creatinine, magnesium, cortisol, total protein, phosphorus, chloride and creatinine kinase isoenzymes (Table 2). The study by Bloom confirmed most of these but did not disclose difficulty with phosphorus, chloride or creatinine kinase. Two types of error are possible. Either the clinical instrument may indicate that the analysis cannot be performed because it has detected an anomaly (for example, "high background absorbance") in the specimen or an incorrect value may be generated. The second of these is clearly of greatest concern because it can 28 Journal of Ophthalmic Photography Vol. 13, No. 1 June 1991 Table 1. Clinical laboratory tests not affected by intravenous fluorescein Alanine aminotransferase Albumin Alkaline phosphatase Amylase Aspartate aminotransferase Bilirubin* Calcium Carbon dioxide Chloride* Cholesterol Creatinine kinase Glucose Lactate dehydrogenase Phosphorus* Potassium Salicylates Sodium Triglycerides Urea *Other studies have reported interference. Results will be methoddependent. (Abstracted from Bloom et al.) lead to errors in clinical interpretation of the tests without the clinician realizing that the test result is in error. The most serious level of interference occurs when the test sample is drawn within minutes after fluorescein injection. Tests such as creatinine and total protein are sensitive to the very high levels of fluorescein at this time, but the interference is no longer present three hours after injection. 5 Interestingly, creatinine measurement at five minutes was shown to be abnormal with the SMAC (Technicon) analytical instrument, but not with other methods such as the ASTRA (Beckman), CΧ3 (Beckman), or ACA (Dupont) instruments. Interference with the magnesium assay on the ACA instrument could be eliminated by changing the wavelength used to spectrometrically determine the value from 510 to 540 nm. This did not eliminate interference with the total protein measurement. The abnormalities which lasted the longest after fluorescein injection were analyses done on the TDx (Abbott) equipment for cortisol, digoxin, quinidine and thyroxine. Abnormalities persisted in some samples for up to 12 hours after injection. 5 Interestingly, every Table 2. Clinical laboratory tests which may be affected by intravenous fluorescein Creatinine Cortisol* Digoxin* Magnesium Quinidine* Creatinine Thyroxine* Total protein *Interference may persist for 3 to 12 hours after injection. sample evaluated did not have the same degree of interference for each test and therefore some values can be correct in the presence of fluorescein whereas others may be incorrect at a similar interval after injection with differences between individuals. This may be related to individual differences in metabolism and excretion of fluorescein.