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CAP Home > CAP Reference Resources and Publications > CAP TODAY > CAP TODAY 2009 Archive > Q and A
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  Q & A

 

 

 

 

May 2009

Editor:
Fredrick L. Kiechle, MD, PhD

Question Q. What is the reference range for pH of the eye? Can you refer me to an article that describes emergency-room procedures for chemical splashes to the eye?

A. There are three fluid components in the eye. The components are, from anterior to posterior, the tear film maintained by the lacrimal glands, the aqueous humor produced by the ciliary processes, and the vitreous humor, which is a secretion product from vessels, fibroblasts and hyalocytes, and the retina. Each of these fluids has a broad range of reported physiologic pH from human and animal studies, and all serve in some buffering capacity.

The tear film bathes the front of the eye, namely, the cornea and conjunctiva. The electrolyte composition of the tear film varies with flow rate, going from hypertonic to isotonic with slow to faster flow rates.1 Tear pH measured in normal human subjects is highly variable, ranging from 6.5 to 7.6, with a mean value of 7.0.2 Individual diurnal variations are also reported, with decreases in pH associated with prolonged periods of eye closure.3

The aqueous humor is located in the anterior chamber of the eye, between the cornea and lens. The aqueous humor has a physiologic pH of approximately 7.38.4 Numerous rabbit model studies have reported a range of pH values from 7.38 to 7.6.5

The vitreous humor, a bulky transparent gel between the lens and posterior retina, is composed mainly of water (99 percent), hyaluronic acid, and collagen. The pH of the vitreous humor has been measured in normal pig eyes, with an average pH of 7.29 seen with a mean arterial pH of 7.41.6

Chemical splashes to the eye are a frequent cause of work-related injuries, with alkali burns more common and dangerous to eye structures than acidic burns because of their deeper penetration. Within a few minutes after a chemical burn, there is a change in the pH of ocular tissues, which is thought to correspond to the severity of subsequent damage. It should be noted that the pH of the tear film immediately following irrigation will often be normal and can be falsely reassuring. Alkali chemicals may take several minutes to diffuse from the anterior chamber, causing the pH to rise after irrigation has been discontinued. Eye rinses with buffering solutions have been suggested as more effective than tap water or saline.7 A 2008 article by Spector and Fernandez8 discusses emergency management and acute treatment procedures.

Typically, the pH of tear fluid is measured using litmus paper. Strictly speaking, this test constitutes point-of-care testing and is therefore subject to regulations under CLIA ’88. In practice, the test is rarely performed and generally falls under the radar of laboratory and hospital surveyors. The major problems we have encountered with pH measurements of fluids with litmus paper are as follows: 1) expired or improperly stored litmus paper supplies are used, 2) operator training is poor and procedures are not standardized, 3) litmus paper products that have never been validated for use on human specimens are used, and 4) there is noncompliance with basic requirements under CLIA ’88.

For these reasons, if a hospital or clinic is planning to perform tear pH testing, we recommend that the point-of-care testing program oversee the test’s performance to ensure minimally acceptable compliance with federal regulations and good laboratory practice.

References

  1. Gilbard JP, Dartt DA. Changes in rabbit lacrimal gland fluid osmolarity with flow rate. Invest Ophthalmol Vis Sci. 1982;23:804–806.

  2. Abelson MB, Udell IJ, Weston JH. Normal human tear pH by direct measurement. Arch Ophthalmol. 1981;99:301.

  3. Carney LG, Hill RM. Human tear pH. Diurnal variations. Arch Ophthalmol. 1976; 94:821–824.

  4. Edelhauser HF, Ubels JL. Cornea and sclera. In: Kaufman PL, Alm A, eds. Adler’s Physiology of the Eye. 10th ed. U.S.: Mosby; 2003:89.

  5. Kompa S, Redbrake C, Hilgers C, et al. Effect of different irrigating solutions on aqueous humour pH changes, intraocular pressure, and histological findings after induced alkali burns. Acta Ophthalmol Scand. 2005;83:467–470.

  6. Andersen MV. Changes in the vitreous body pH of pigs after retinal xenon photocoagulation. Acta Ophthalmol (Copenh). 1991;69:193–199.

  7. Rihawi S, Frentz M, Schrage NF. Emer-gency treatment of eye burns: which rinsing solution should we choose? Graefes Arch Clin Exp Ophthalmol. 2006;244:845–854. Epub 2005 Dec.

  8. Spector J, Fernandez WG. Chemical, ther-mal, and biological ocular exposures. Emerg Med Clin North Am. 2008;26:125–136, vii.

JiYeon Kim, MD, MPH
Clinical Pathology Resident
Clinical Chemistry Laboratory
Massachusetts General Hospital
Boston

Kent Lewandrowski, MD
Associate Chief of Pathology
(Director of Clinical Services,
Anatomic and Clinical Pathology) Massachusetts General Hospital
Associate Professor of Pathology
Harvard Medical School
Boston

Question Q. We frequently receive subpoenas for blood specimens drawn for alcohol determination for medical concerns. Are there guidelines for releasing specimens to proper police authorities for example, time specimen must be held in the laboratory?

A. To the best of my knowledge, there are no universal requirements or guidelines, either legal or regulatory, that apply to this specific situation at the state or federal level. The hospital or laboratory, or both, should have a clearly stated policy about specimen retention for medical blood alcohols just as they should for any other orderable laboratory test. In this case, the policy should state how many days residual specimen is to be retained in the laboratory and how soon after testing it can be released from the lab pursuant to a subpoena.

The specimen of choice for ethanol determination is blood, and because of the volatile nature of the analyte, specimens must be capped to avoid outgassing due to evaporation.1 According to Wu: “Sodium fluoride, 1% (w/v) is the most satisfactory preservative. Blood specimen without sodium fluoride is reliable at 25°C up to 2 d, at 5°C up to 2 wk, and at –15°C up to 4 wk. With sodium fluoride preservative, specimen is reliable at 25°C up to 2 wk, at 5°C up to 3 mo, and at –15°C up to 6 mo.”2 Therefore, properly obtained and preserved (that is, capped fluoride tube, refrigerated or frozen) blood specimens do not have to be released immediately just because they have been subpoenaed, if they are needed for such things as other critical add-on testing. Patient care needs should take precedence. On the other hand, they should be released as early as the lab deems feasible, since repeat or confirmatory testing, or both, necessitated by the lack of specificity in the ADH enzymatic method typically used for medical blood alcohols in hospitals, is generally what motivates such subpoenas.

Medico-legally, what is critical for the laboratory is that all of this must be in writing, in the form of a standard operating procedure that takes all of the preceding considerations into account. The legal aspects of such cases typically include concern about the absence of chain of custody for “medical” (as opposed to “legal”) blood alcohol specimens and the implications for the accuracy of test result reporting. In such cases, I have been asked, as toxicology director, to testify what I estimate the probability is of a mislabel or misidentification of a blood specimen in my hospital. Using other general QA data, which we routinely record, I’ve been able to estimate this probability at approximately one in 10,000 specimen transactions at my institution. This is a sufficiently low number such that defense objections to chain-of-custody problems are typically over-ruled.

References

  1. Burtis CA, Ashwood ER, Bruns DE. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 4th ed. St. Louis, Mo.: Elsevier Saunders; 2006:1302–1303.

  2. Wu AH. Tietz Clinical Guide to Laboratory Tests. 4th ed. St Louis, Mo.: Elsevier Saunders; 2006:1345.

Michael G. Bissell, MD, PhD, MPH
Professor of Pathology and Director of Clinical Chemistry and Toxicology
Ohio State University Medical Center
Columbus

Member, CAP Toxicology Resource Committee


Dr. Kiechle is medical director of clinical pathology, Memorial Healthcare, Hollywood, Fla.
 
 
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