COLLECTION AND TRANSPORT OF SPECIMENS

Sterile, Dacron, or Rayon swabs with plastic shafts are suit able for collecting most upper respiratory tract microorganisms. Flocked swabs may also be used when available. If the swab remains moist, no further precautions need to be taken for specimens cultured within 4 hours of collection. After that period, transport medium is required to maintain viability and prevent overgrowth of contaminating organisms. Swabs for detection of group A streptococci (Streptococcus pyogenes) are the only exception. This organism is highly resistant to desiccation and remains viable on a dry swab for as long as 48 to 72 hours. These throat swabs can be placed in glassine paper envelopes for mailing or transport to a distant laboratory.

Throat swabs are also adequate for recovery of adenoviruses and herpes viruses, Corynebacterium diphtheriae, Mycoplasma, Chlamydia, and Candida spp. Recovery of C. diphtheriae is enhanced by culturing both the throat and nasopharynx.

Nasopharyngeal swabs are better suited for recovery of Bordetella pertussis, Neisseria spp., along with several viruses including respiratory syncytial virus, parainfluenza virus, and the other viruses causing rhinitis. Optimum conditions for the collection and transport of specimens for viral detection or culture are described in Chapter 65. Although swabs made of calcium alginate are commonly used to collect nasopharyngeal specimens (excluding those specimens for chlamydia or viral culture), nasopharyngeal secretions collected by either aspiration or washing will improve recovery for Bordetella pertussis because a larger amount of material is obtained.

The type of swab used for collection is very important. For example, cotton swabs should never be used for culture because fibers contain fatty acids on the surface, which are capable of killing Bordetella. Calcium alginate or Dacron swabs are acceptable for obtaining nasopharyngeal swab specimens, with calcium alginate being optimal for culture. However, if polymerase chain reaction (PCR) is to be performed, Dacron or rayon swabs on plastic shafts are preferred.

Specimens for B. pertussis ideally should be inoculated directly to fresh culture media at the patient’s bedside. If this is not possible, transport for less than 2 hours in 1% Casamino acid medium at room temperature is acceptable. If specimens are plated on the day of collection, Amies transport medium with charcoal is acceptable. If specimens are plated more than 24 hours after collection, Regan-Lowe or Jones-Kendrick transport medium is optimal; both contain charcoal, starch, and nutrients as well as cephalexin. If lengthy delays in transport are expected, transport of specimens in Regan-Lowe medium at 4°C is recommended.

DIRECT VISUAL EXAMINATION OR DETECTION

A Gram stain of material obtained from upper respiratory secretions or lesions may not improve diagnosis. Yeast-like cells can be identified, which are helpful in identifying thrush, and the characteristic pattern of fusiform and spirochetes of Vincent’s angina may be visualized. Gram’s crystal violet (allowed to remain on the slide for 1 minute before rinsing with tap water) and the Gram stain can be used to identify the spirilla and fusiform bacilli of Vincent’s angina. However, if crystal violet is used, the smear should be very thin because everything will be intensely Gram positive, making a thick smear difficult to read. Additionally, spirilla and bacilli may be stained using a dilute solution of carbol fuchsin.

For causes of pharyngitis, Gram stains are unreliable. Direct smears of exudate from membrane-like lesions used to differentiate diphtheria from other causes are also not reliable or recommended.

Fungal elements, including yeast cells and pseudohyphae, may be visualized with a 10% potassium hydroxide (KOH) preparation, calcofluor white fluorescent stain, or periodic acid-Schiff (PAS) stain. Direct examination of material obtained from the nasopharynx of suspected cases of whooping cough using a fluorescent antibody stain has been shown to yield some early positive results for detection of B. pertussis. However, direct fluorescent antibody (DFA) staining of nasopharyngeal secretions often lack sensitivity and specificity depending on the antibody used. Numerous studies have demonstrated that PCR-based assays for B. pertussis in nasopharyngeal secretions are superior to both DFA and culture. Various methods, including fluorescent antibody stain reagents, enzyme immunoassays, and nucleic acid amplification methods are also commercially available to detect numerous viral agents.

Improvement in the development of rapid methods for detection of group A streptococcal antigen or nucleic acid has obviated the need for culture of pharyngeal specimens. At least 40 commercial products are available to identify group A streptococcal antigens using membrane enzyme immunoassays or liposomal and optical immunoassay techniques. Although the specific procedures vary with the products, several generalizations can be made. Throat swabs are incubated in an acid reagent or enzyme to extract the group A specific carbohydrate antigen. Dacron swabs seem to be most efficient at releasing antigen, although other types of swabs may yield accept able results. In laboratory comparisons between a rapid antigen method and conventional culture methods for detecting the presence of group A streptococci in throat swabs, the commercial kits have shown relatively accept able (62% to more than 90%) sensitivity and specificity.

Specimens with a negative direct antigen test for group A streptococci should be cultured (requires collection of specimen with two swabs) or confirmed using a nucleic acid method. Group A streptococci can be directly detected from pharyngeal specimens by nucleic acid testing using different molecular assay formats. The commercially avail able assay (Probe Group A Strep Direct Test (GAS Direct), Hologic-GenProbe, Inc., San Diego, California) that employs a nonisotopic, chemiluminescent, single-stranded DNA probe complementary to the rRNA target of the group A Streptococcus. The assay detects organisms directly from swab specimens by lysing the bacterial cells before amplification. Dacron swabs are acceptable for use with this assay. Sensitivities of the Gen-Probe Group A Strep Direct Test range from 91.7% to 99.3% when compared with culture.

A rapid-cycle real-time PCR method, the Light Cycler Strep-A (Roche Applied Science, Indianapolis, Indiana), also detects S. pyogenes directly from throat swabs. Using this technology, 32 samples (including controls) can be tested per run in about 1.5 hours. Isothermol DNA amplification is also available for the detection of Group A Streptococcus from throat swabs (Illumigene Group A Streptococcus, Meridian Bioscience, Inc., Cincinnati, Ohio) and demonstrates sensitivity equal to the Group A Strep Direct test. See Chapter 8 for more information on isothermal DNA amplification.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم صناعة الشبابيك يباشر أعمال إدامة شباك مقام الإمام المهدي (عجّل الله فرجه)

المجمَع العلمي ينظم ندوته القرآنية الأسبوعية في جامعة الكوفة

جامعة العميد تقيم الملتقى العلمي الثاني لخريجي كلية طب الأسنان

ضمن دورتها العلمية (في رحاب العقيدة) الهيأة العليا لإحياء التراث تقدم محاضرة عن مفهوم الأسبقية في الإسلام

المزيد

الآخبار الصحية

اللحظات الأخيرة قبل الموت.. هذا هو تسلسل "فقدان الحواس"

وخز الأطراف وضعف الذاكرة.. علامات على نقص فيتامين هام

عادة صباحية بسيطة.. تأثيرها قد يحسن ضغط الدم

عادات يومية تدمر قلبك بصمت.. خبراء يطلقون التحذير

المزيد

الآخبار التكنلوجية

الذكاء الاصطناعي يفك لغز آثار أقدام الديناصورات

القمر أولا.."سبيس إكس" تؤجل خططها الخاصة بالرحلات إلى المريخ

الذكور أم الإناث؟ دراسة "تنسف الشائع" بشأن التوحد

خبير يكشف عن اقتراب العلماء من فك شفرات "لغات الحيوانات" !

المزيد

مواضيع ذات صلة

Differential Diagnosis of Pancytopenia

Diagnosis of Aplastic anemia

tuberculosis testing (TB testing, Interferon gamma release assay [IGRA, T-Spot.TB], QuantiFERON-TB Gold [QFT, QFT-G, TB Gold test, TB blood test], Nucleic acid amplification for TB [NAAT], TB antibody)

thyrotropin-releasing hormone stimulation test (TRH stimulation test, Thyrotropin-releasing factor stimulation test [TRF stimulation test])

Laboratory Diagnosis of Lower Respiratory Tract Infections: Specimen Processing

Laboratory Diagnosis of Lower Respiratory Tract Infections: Specimen Collection and Transport

thyroid scanning (Thyroid scintiscan)

thyroid fine needle aspiration biopsy (FNAB, Skinny needle thyroid biopsy, Fine-needle aspiration [FNA])

thoracentesis and pleural fluid analysis (Pleural tap)

The Role of Laboratory in Urgency: Altered State of Consciousness

Preoperative Laboratory Test

sputum culture and sensitivity (C&S, Culture and Gram stain)