The Prolex™ Streptococcal Grouping Latex Kit (PL.030) identifies Lancefield groups A, B, C, D, F, and G beta-hemolytic streptococci in about 60 seconds from a fresh colony. Published evaluations report approximately 99% sensitivity and 100% specificity versus conventional Lancefield serology. Group A (S. pyogenes) drives pharyngitis, scarlet fever, and necrotizing fasciitis; Group B (S. agalactiae) is the leading cause of neonatal sepsis. Latex agglutination replaces hot acid extraction and antiserum precipitin panels with a single 2-minute card test.
Key Facts
- Groups detected: Lancefield A, B, C, D, F, and G — the clinically actionable beta-hemolytic streptococci.
- Time to result: ~60 seconds of rocking; full workflow under 2 minutes from colony to group call.
- Performance: ~99% sensitivity and 100% specificity vs. conventional Lancefield serology in published evaluations.
- Format: Disposable reaction card + enzymatic extraction reagents + six color-matched latex bottles + positive control.
- Throughput: Up to 50 tests per kit, suitable for routine clinical microbiology, antenatal GBS screening, and outbreak investigation.
- Manufacturing: CE-marked IVD, ISO 13485:2016, manufactured by Pro-Lab Diagnostics.
Overview
Beta-hemolytic streptococci look the same on a blood-agar plate: small colonies surrounded by a clear zone of complete hemolysis. The clinical implications, however, depend entirely on which Lancefield group the isolate belongs to. Group A streptococcus drives pharyngitis and invasive disease; Group B is the leading cause of neonatal sepsis; Groups C, F, and G cause a growing share of bacteremia and soft-tissue infections in adults. Distinguishing them quickly at the bench is one of the highest-value tests in routine clinical microbiology.
The Prolex™ Streptococcal Grouping Latex Kit does this in about 60 seconds. Latex microspheres coated with group-specific antibodies are mixed with an enzymatically extracted bacterial suspension; if the matching cell-wall carbohydrate antigen is present, the latex agglutinates into a visible clump that any bench technologist can read against a dark background.
A short history: Lancefield groups A through G
In the 1930s, Rebecca Lancefield at the Rockefeller Institute classified streptococci by the immunoreactivity of a polysaccharide antigen in the bacterial cell wall — the “group antigen” or C-substance. Her serological scheme assigned letters A through V; in clinical microbiology the relevant set has narrowed to A, B, C, D, F, and G:
- Group A — Streptococcus pyogenes: pharyngitis (“strep throat”), scarlet fever, impetigo, erysipelas, cellulitis, streptococcal toxic shock syndrome, necrotizing fasciitis, and the post-infectious sequelae acute rheumatic fever and post-streptococcal glomerulonephritis.
- Group B — Streptococcus agalactiae: the leading cause of neonatal sepsis and meningitis. CDC and ACOG recommend universal antenatal GBS screening at 36 0/7–37 6/7 weeks and intrapartum antibiotic prophylaxis for colonized mothers.
- Group C and Group G: typically the large-colony-forming Streptococcus dysgalactiae subsp. equisimilis, increasingly recognized in pharyngitis, bacteremia, and skin/soft-tissue infections.
- Group D: historically the enterococci (now their own genus) and the non-enterococcal S. bovis group, the latter associated with colorectal malignancy when isolated from blood.
- Group F: the “Streptococcus anginosus group” (formerly “milleri”), notorious for forming abscesses in the brain, liver, and deep soft tissue.
For every one of these, the first step after a beta-hemolytic colony appears on the plate is the same: what Lancefield group is this?
How the latex agglutination chemistry works
The Prolex™ kit ships six latex reagent bottles, one per group. Each contains microspheres covalently coupled to antibodies raised against the corresponding Lancefield group polysaccharide. The procedure has three operational steps:
- Enzymatic antigen extraction. The kit's Extraction Reagents 1 and 2 release the group carbohydrate from the streptococcal cell wall. Unlike legacy hot-acid (Lancefield) or Fuller's formamide methods, the enzymatic extraction takes seconds at room temperature and is gentle enough that bench staff do not need a fume hood.
- Mixing with the six latex reagents. One drop of extract is placed into each of six reaction circles on the disposable card; one drop of the corresponding latex (A through G) is added on top. A clean mixing stick spreads the reaction across the circle.
- Agglutination read. The card is rocked by hand for up to 60 seconds. A positive reaction is a coarse, visible clumping of the latex; a negative reaction stays as a smooth, milky suspension. A pure beta-hemolytic isolate should agglutinate one and only one of the six latex reagents.
Performance: 99% sensitivity, 100% specificity
Published method-comparison studies of commercial latex grouping kits against the conventional Lancefield serological reference report approximately 99% sensitivity and 100% specificity. The specificity is not surprising: the bound antibodies are highly group-specific, and false positives are vanishingly rare when the kit is used on pure beta-hemolytic colonies. The remaining ~1% sensitivity gap is dominated by non-groupable variants and isolates with very low group-antigen expression — situations in which any antigen-based method, including conventional precipitin, also struggles.
The practical effect at the bench is that a positive Prolex™ result is, for routine clinical purposes, definitive for group assignment.
Why labs replaced conventional serology
Conventional Lancefield grouping — capillary precipitin, counter-immunoelectrophoresis, or Ouchterlony double-diffusion — requires hot HCl or formamide extraction of the group antigen, a panel of group-specific antisera, and a trained eye to read the precipitin line at the antiserum–antigen interface. The full procedure can take 1–2 hours per isolate, and the antisera are expensive and short-dated.
Latex agglutination collapses all of that into a 60-second card test that any MLT can run on the first day of training. The reagents are stable for the full shelf life at 2–8°C, the equipment requirement is a clean bench and good lighting, and the result is unambiguous: clump or no clump.
“For routine identification of beta-hemolytic streptococci to group level, latex agglutination has replaced conventional serology in essentially all clinical microbiology laboratories.”
Where Prolex™ fits in the wider workup
The Prolex™ kit is for organisms that already look like beta-hemolytic streptococci on the plate — gram-positive cocci in chains, catalase-negative, beta-hemolytic on sheep blood agar. The upstream and downstream steps still matter:
- Gram stain and catalase first. Pro-Lab's Pro-Stains Gram stain confirms gram-positive cocci in chains; a negative oxidase/catalase rules out the staphylococci.
- Differentiate from Staphylococcus aureus. If a colony is catalase-positive, use the Prolex™ Staph Kit instead — it detects clumping factor, protein A, and capsular polysaccharides for S. aureus and MRSA.
- Archive the isolate. Any clinically significant streptococcus belongs in the laboratory's reference collection. Microbank® beads preserve viability at -20°C, -70°C, or -80°C with up to ~25 retrievals per vial.
Clinical and research applications
Beyond the diagnostic lab, Prolex™ is used in:
- Antenatal Group B screening: rapid confirmation of S. agalactiae from selective broth or chromogenic agar, supporting intrapartum prophylaxis decisions.
- Outbreak and surveillance work: public-health laboratories use latex grouping to triage isolates before molecular emm-typing of invasive Group A strep.
- Veterinary diagnostics: S. dysgalactiae and S. equi identification in equine and bovine clinical samples.
- Research and teaching: a clean, reproducible demonstration of antibody-antigen agglutination chemistry for MLT and medical-laboratory-science programs.
Frequently Asked Questions
What is the Prolex™ Streptococcal Grouping Latex Kit?
The Prolex™ Strep Grouping Latex Kit (PL.030) is a rapid latex agglutination test that identifies Lancefield groups A, B, C, D, F, and G from beta-hemolytic streptococcal colonies in 60 seconds. Latex particles coated with group-specific antibodies bind the matching cell-wall carbohydrate antigen and form a visible clump when the antigen is present.
How accurate is latex agglutination for Lancefield grouping?
Published evaluations of commercial latex grouping kits report approximately 99% sensitivity and 100% specificity when compared with conventional Lancefield serology, which is the historical reference standard. Specificity is consistently 100% because the bound antibodies are highly group-specific.
How long does the Prolex™ kit take to return a result?
The full procedure — antigen extraction, mixing, and reading — takes about 2 minutes from a fresh colony. The agglutination reaction itself is read at 60 seconds.
Why is Group A streptococcus clinically important?
Group A streptococcus (Streptococcus pyogenes) causes pharyngitis (strep throat), scarlet fever, impetigo, cellulitis, streptococcal toxic shock syndrome, and necrotizing fasciitis. Rapid identification from a beta-hemolytic isolate guides antimicrobial therapy and supports infection-control reporting.
Why is Group B streptococcus clinically important?
Group B streptococcus (Streptococcus agalactiae) is the leading cause of neonatal sepsis and meningitis. Identifying GBS from antenatal screens or neonatal isolates allows intrapartum antibiotic prophylaxis and prompt neonatal treatment.
What is the advantage over conventional serology?
Conventional Lancefield serology — capillary precipitin or counter-immunoelectrophoresis — requires hot acid extraction, antiserum panels, and 1–2 hours of bench time. Latex agglutination delivers the same group assignment in 60 seconds at the bench, with no precipitin reading expertise required.
For more information about the Prolex™ Streptococcal Grouping Latex Kit, contact info@pro-lab.us or visit the Prolex™ Strep Grouping Kit product page to order online.