Laboratory Medicine Updates - November 30, 2005

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LABORATORY MEDICINE UPDATE

November 30, 2005

Update From the Cytogenetics Lab – Na Heparin Green Top Tubes

The 2 mL and 7 mL Na Heparin green top vacutainer tubes are no longer available.  Only a 4 mL tube is available from BD Vacutainers.  For most conventional karyotype studies, only one tube of blood is necessary.  If multiple studies (e.g., conventional cytogenetics and breakage analysis) are requested, a second tube should be drawn and sent.  The 4 mL tubes are suitable for bone marrow aspirates.

 

Coagulation Reference Interval Updates

Due to a change in reagents, the aPTT (PTT) reference interval is now 23.5 to 34.8 seconds.  This applies to both the PTT and HPSORB test codes.

The new heparin therapeutic range for the heparin PTT test (HEPPTT test code) is 60 to 93 seconds, which corresponds to 0.3 – 0.7 Anti-Xa units/mL.

 

HIV Genotyping

On November 9, the UVa Molecular Diagnostics Laboratory began performing HIV Genotyping (Test Code: HIVG) using the same FDA-approved method (Bayer TruGene) as performed by our referral laboratory (Mayo).  Testing can only be performed on samples with HIV viral loads greater than 1000 copies/ml.  If a HIV viral load has not been performed within 2 weeks, a HIV Viral load must be ordered and performed on the sample prior to HIV Genotyping.

The required specimen for testing is EDTA (lavender top) blood and at least 0.2 ml of plasma is needed for PCR sequencing analysis.   HIVG is MIS orderable.   The turn around time is two weeks and a final report will be faxed to the submitting physician.   At a future date, reports will be available in CoPath.

The Bayer TruGene resistance interpretation is based upon phenotypic and virologic response data (The Consensus Panel) for correlation of Protease and RT sequences to antiretroviral drug resistance.   Concurrent with this change, PhenoSense testing (a miscellaneous send-out) will only be available with prior approval by the on-call clinical pathology resident. The charge for HIV Genotyping is $631.00.   Please call the laboratory at 982-3310 with questions.

 

Troponin/CKMB Reflex Test No Longer Available

The reflex test of CKMB following an elevated Troponin I determination is no longer available.  The two tests must be ordered individually if both results are needed.

 

Expanded Immunofluorescent Analysis of Renal Biopsies

The α 3-5 chains of type IV collagen are found in only a few sites in the body, including renal glomerular basement membranes.  In Alport syndrome, a disease caused by over 300 different genetic mutations, various defects in the α 3, 4 or 5 chains lead to renal failure.  The majority of cases of Alport syndrome are X-linked and have abnormalities of the α 5 subunit, with far fewer autosomal recessive cases and rare autosomal dominant ones.  The autosomal recessive form is associated with mutations of the α 3 and 4 collagen IV subunits.  Alport syndrome predominantly affects men producing progressive renal failure with sensorineural hearing loss and lenticonus of the anterior lens capsule.  The majority of women with a heterozygous mutation of collagen IV α 5 have microscopic hematuria and maintain renal function, with a minority going on to renal failure in late adulthood.

Thin basement membrane nephropathy, also erroneously known as benign familial hematuria, occurs in approximately 1% of the population and was thought for many years to be a benign disease with no risk for deterioration of renal function.  Many cases demonstrate an autosomal dominant mode of inheritance.  It is now understood that over 1/3 of these patients have heterozygous α 3/4 mutations, but lose kidney function no more rapidly than the general population.  Homozygosity or mixed heterozygosity on the other hand, can cause autosomal recessive Alport syndrome resulting in early loss of kidney function.

Cases of adult-onset Alport syndrome may be misdiagnosed as thin basement membrane nephropathy, as clinical, histologic and even ultrastructural findings cannot reliably distinguish between the two.  Thin glomerular basement membranes are one of the features of Alport syndrome on ultrastructural analysis and may be the only finding in females.  Genetic diagnosis is extremely complex and not readily available.  Antibodies for indirect immunofluorescent staining of the α 3-5 chains of Type IV collagen are now available commercially and can be used to diagnose many cases of Alport disease, even those of female carriers.  Skin biopsy is the recommended screening method for relatives of patients with Alport syndrome who have hematuria.  Immunofluorescence is performed on renal biopsies if the clinical history and/or electron microscopic findings are suggestive of Alport syndrome.

A recent satellite conference of the Renal Pathology Society at the 2005 USCAP meeting in San Antonio was entirely devoted to Alport syndrome and thin basement membrane nephropathy.  The consensus was that differential collagen staining should be done on all cases of thin basement membrane nephropathy, as well as on Alport cases.  Antibodies for α 1/2 (used as positive controls as present in all basement membranes), and α3/5 are available from Wieslab , Sweden , and are straightforward to use.  The antibodies were originally validated at the Mayo Clinic and are considered the gold standard.  With the introduction of these additional immunofluorescent antibodies to the panel currently applied to renal biopsies at UVA, diagnosis of cases falling into this overlap category between Alport syndrome and thin basement membrane disease is greatly enhanced.