Prenatal and postnatal diagnostic testing
Klinefelter syndrome may be diagnosed prenatally from fetal cytogenetic analyses performed on chorionic villi or amniocytes. Noninvasive prenatal tests (NIPT) that analyze cell-free fetal DNA circulating in maternal blood are also available; however, there is limited data on detection of Klinefelter syndrome and other sex chromosome aneuploidies. 
If Klinefelter syndrome is not diagnosed prenatally, a patient with 47,XXY karyotype may demonstrate various subtle, age-related clinical signs that would prompt diagnostic testing. These include the following  :
Infants - Hypospadias, small phallus, cryptorchidism (undescended testes)
Toddlers - Developmental delay (especially expressive language skills), hypotonia
Elementary school–aged boys - Language delay (especially expressive language skills), learning disabilities, delayed social development, pervasive developmental disorder (PDD-NOS), mild autism spectrum disorder, attention deficit disorder (ADD)/attention deficit hyperactivity disorder (ADHD)
Older boys and adolescent males - Tall stature; delayed or incomplete pubertal development with eunuchoid body habitus; gynecomastia; small, firm testes; sparse body hair
Adults - Testicular failure, significantly diminished or absent sperm count and/or breast malignancy, dental problems, delay in vocational success, anxiety and/or depression, mood disorders
Karyotype analysis on peripheral blood lymphocytes, the XCAT-KS buccal swab test, fluorescence in-situ hybridization (FISH), and microarrays are options for postnatal diagnostic testing.
The 47,XXY karyotype is found in 80-90% of males who are diagnosed with Klinefelter syndrome. About 10% of patients have mosaicism (more than one cell line): 46,XY/47,XXY; 46,XY/48,XXXY; and 47,XXY/48,XXXY. Remaining cases include karyotype variants such as 48,XXYY; 48,XXXY; 49,XXXYY; and 49,XXXXY. About 1% of cases are due to a structurally abnormal X in addition to a normal X and Y, such as 47,X,i(Xq)Y and 47,X,del(X)Y.
Androgen receptor gene quantitative real-time polymerase chain reaction (AR-qPCR) technique
This is a simple and reliable screening method for the diagnosis of patients with Klinefelter syndrome or other chromosomal disorders involving an aberrant number of X chromosomes. 
The screening method determines the copy number assessment of the androgen receptor (AR) gene, located at Xq11.2-Xq12.
From childhood with progression to early puberty, the pituitary-gonadal function observed is within normal limits for 47,XXY males. 
At midpuberty and later, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) concentrations rise to hypergonadotropic levels, inhibin B levels fall until they are undetectable, and testosterone levels are at low or low-normal levels after an initial increase. Hence, most adult males with Klinefelter syndrome have hypergonadotropism with varying degrees of androgen deficiency. 
A complete hormonal evaluation includes plasma levels of FSH, LH, testosterone, estradiol, prolactin, and insulinlike growth factor (IGF)–1. Young males (aged 12-14 years) tend to have high plasma FSH, LH, and estradiol levels and low plasma testosterone levels.
Forty-seven percent of men with Klinefelter syndrome have adrenal steroidogenic deficiency, so cortisol levels should be routinely measured. 
Serum osteocalcin levels are decreased and the hydroxyl-proline:creatinine ratio is increased, reflecting decreased formation and increased resorption of bone.
Urinary gonadotropins are increased due to abnormal Leydig cell function.
Administration of human chorionic gonadotropin (hCG) will increase testosterone levels in men with Klinefelter syndrome, but the response is diminished when compared with the unaffected male population.
Routine bone density screening
Bone density screening is recommended because androgen deficiency significantly increases the risk of osteopenia and osteoporosis.
Men with Klinefelter syndrome have an increased risk of deep vein thrombosis and pulmonary embolism. Consider screening men with 47,XXY or other sex chromosomal aneuploidies for genetic mutations that lead to hypercoagulability states. 
Echocardiography is performed to assess for mitral valve prolapse.
Radiography is performed to assess for bone density, radioulnar synostosis, and dental concerns (taurodontism).
In prepubertal boys with Klinefelter syndrome, testicular biopsies may reveal preservation of seminiferous tubules with reduced numbers of germ cells; Sertoli and Leydig cells appear normal. 
Testicular biopsies of adult males with Klinefelter syndrome are characterized by extensive fibrosis and hyalinization of the seminiferous tubules and hyperplasia of the interstitium; however, the tubules may show residual foci of spermatogenesis. Histologic findings may include small, firm testes with seminiferous tubular hyalinization; sclerosis; and atrophy with focal hyperplasia of mostly degenerated Leydig cells. Germ cells are markedly deficient or absent. 
In Klinefelter syndrome males with mosaicism, a progressive degeneration and hyalinization of seminiferous tubules occurs following puberty, despite the presence of normal-sized testes and spermatogenesis at puberty.
Histology of gynecomastic breasts reveals hyperplasia of interductal tissue.
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