UCSD Cystine Determination Laboratory
Email: cystine@ucsd.edu
NOTE: Patients must come to San Diego for testing.
Cystinosis is a recessive defect. Nevertheless, the presence of the defect in carriers can be assessed. The detection depends upon measurement of cystine in polymorphonuclear(PMN) leukocytes. The PMN leukocyte fraction has a greater cystine/ mg protein than lymphocytes or mixed leukocytes. The sensitivity of the method is thus enhanced. The method for separation of the PMN is critical (Lori Smolin paper). The values were determined by measurements on PMN leukocytes from obligate heterozygotes and presumed random normals.
Individuals at risk for carrier status must come to the San Diego laboratory for testing. Samples prepared elsewhere are so often uninterpretable that we do not recommend the procedure.
Sensitivity -> 126/131 = 96%
True positive correctly identified.
(based on obligate carriers) Specificity -> 46/47 = 98%
True negative correctly identified. (assuming random controls are in fact not carriers) Positive predictive value ->126/127 = 99%
Proportion of positive correctly identified.
(based on obligate carriers) Negative predictive value -> 48/53 = 91%
Proportion of negative correctly identified. (assuming random controls are in fact not carriers)
NOTE: Since we do not know
the TRUE range of normal,
some uncertainty will remain for individuals
with borderline values.
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